I can't figure out why my fish keep dying

[RVARach]

A bit of back story, I purchased this (36 gallon bowfront) tank used from someone selling it on facebook. It was previously used as an aquarium, not for a reptile or anything like that. I thoroughly cleaned the tank with boiling water and a (new) sponge. I took the filter apart and cleaned it as well as soaking it in boiling water. There was aquarium sand in the tank when I first purchased it. I set the tank up and cycled it for approximately 6 weeks. I used filter media from another tank I have and put plants in there. After about a month, I changed out the substrate, thinking maybe it could have been something with the sand.

Introduction of fish at the beginning of April, I purchased 2 dalmatian mollies and 2 algae eaters from a big box pet store. One of the mollies died within a few days. The other molly seemed very stressed after the death of its tankmate, so I moved it into my 20 gallon with 2 platys and a lyretail molly (not the smartest move, I know). The dalmatian molly is still alive and well today. Soon after the death of the first molly, the 2 algae eaters died, leaving the tank empty. None of the fish (other than the molly I moved to another tank) survived longer than 2 weeks and none of them have lived any longer than that since. I test my water religiously and had the water tested at both big box pet stores as well as a local fish store. Every time the water has been tested, all the levels have been ideal. The only slight level variation was the pH was elevated (8.4). I used Seachem Neutral regulator for a few days until the levels returned to normal.

I am at my wits end and am devastated to be losing so many fish. I should note that I have 2 other tanks, a 20 gallon planted tank and a 10 gallon tank. I use the same water source and same chemicals in all 3 tanks. All of the plants I have ordered have been split between the 36 gallon and the 20 gallon. I have never run into an issue like this and I am hoping I can get some help. I was having this problem with the fish dying before and after the introduction of co2.

1. Size of tank?
36 gallon bowfront

2. Parameters (Tetra EasyStrips 6 in 1)
Nitrate: 0
Nitrite: 0
GH: 75
Chlorine: 0
Total Alkalinity: 180
pH: 7.8

3. Temperature?
- 77°F

4. FW (fresh water) or BW (brackish)?
- Freshwater

5. How long the aquarium has been set up?
Set the tank up in the beginning of February, first introduced fish in early April.

6. What fish do you have? How many are in your tank? How big are they? How long have you had them?
Sadly, I have probably gone through about 20 fish in this tank ranging from tiger barbs, mollies, platys, bristlenose plecos, red eye tetras, cherry barbs, debauwi catfish, algae eaters and black skirt tetras. Right now, the only two living fish are 2 platys about 1.5 inches long.

7. Were the fish placed under quarantine period (minus the first batch from the point wherein the tank is ready to accommodate the inhabitants)?
The fish were never placed in quarantine because the tank was empty each time I added a new group.

8. a. Live plants:
-creeping charlie
-Bacopa Monnieri
- Alternathera Rosanervig
-Micro sword
-Red Tiger Lotus
- Rotala Rodundifolia Red
- Dwarf Hairgrass
- Rotala Walichii
- Amazon Sword
-Java Fern
-Hygrophila Pinnatifida

b. Sand, gravel, barebottom?
-started with sand
then changed to:
- Carib sea eco complete with gravel on top

c. Rocks, woods, fancy decors? Any hollow decors?
no decor

d. additional equipment
-infused co2 (5lb tank)
-pH drop checker
-14 inch bubbler bar

9. a. Filtration?
- Top Fin silentstream 40. I will usually rinse and reuse the filter with each water change in the siphoned tank water.
b. Heater?
-200 w heater

10. a. Lighting schedule? What lights are used?
- 11am - 10pm
- The light is a Beamswork DA 6500k
b. Any sunlight exposure? How long?
-very little if any

11. a. Water change schedule?
-Every 2 weeks, most recent being Thursday (June 11th)
b. Volume of water changed?
-~20-25% (about 8 gallons)
c. Frequency of gravel/sand (if any) vacuumed?
- spot vacuum once a month

12. Foods?
- frozen brine shrimp
How often are they fed?
-nightly, a very small piece typically consumed in under 5 minutes

13. a. Any abnormal signs/symptoms?
-The fish will be fine and swimming happily soon after they are in the tank. There is no consistency with their survival rate. Some last less than one day, others will last a couple of weeks. Some of the fish will be just fine when I leave for work, and by the time I get home 9 hours later, they are dead at the bottom of the tank. Some will start surfacing to get air then begin swimming listlessly and end up laying at the bottom of the tank barely breathing, sometimes even getting stuck on the filter intake.

b. Appearance of poop?
- Long green strings
c. Appearance of gills?
-Gills look fine, no spots or flaking

14. a. Have you treated your fish ahead of diagnosis?
-I have treated the water, not the fish. I will use a dose of paragard when I initially add fish to the tank.
b. What meds were used?
- No meds, but with each water change, I will dose the tank with Seachem stability and Seachem Neutral Regulator. Weekly dose of Seachem Flourish

15. Insert photos of fish in question and full tank shot if necessary.

 

[TwoTankAmin]

When you say you cycled the tank for 6 weeks, exactly what do you mean. If there were no fish or plants in it and you were not doing something to add ammonia, then you did not cycle anything and in fact may have lost some of the bacteria in the cycled media.

Plants will process ammonia, but they need to be in the tank for a while before you add fish. This gives the plants a chance to settle in and become established,

I am not a fan of test strips unless they are the very expensive lab grade types. You gave no information on ammonia. Fish having trouble breathing is often a sign of issues with nitrite. You say you have none, but I am not so sure, especially with a multi-parameter strip.

I am not a fan of Stability and believe in a truly cycled tank it does nothing good and may do some harm. I am also not big on buffers. Paraguard is a medication. Also,

Paragard IUD is the only birth control that's 100% hormone free, over 99% effective, & works with 1 simple active ingredient.

Your gravel is really the wrong size for plants. These need a much smaller sized gravel or sand to be happy. Water changes should be weekly and 50%, this is good both for both the fish and the plants which also need some macronutrients since there are not enough fish to supply them.

 

[NoodleCats]

The lack of nitrates is suspicious, I suspect the same as above... tank isn't truly cycled.

 

[the loach]

2 platies in a planted 36g, no nitrates are not suspicious... but then the strips aren't great, you need a (liquid) ammonia test as well. From this information, it can't be deterimined the tank is cycled or not.
Tell us more about the dates you added the fish, their numbers, and when you found them dead.
What kind of fish were the 'algae eaters'?
Tank looks nice. But you have to get rid of the bubbler bar/air pump, you can't have that together with CO2.
Is by any chance the air pump on a timer and goes off at night?

I am not a fan of Stability and believe in a truly cycled tank it does nothing good and may do some harm.

So it's not only a fake product, but one that is detrimental to fish as well? That seems pretty out there.

 

[RVARach]

Thank you all for the input. I forgot to mention the ammonia, I have the API liquid test kit for ammonia and my ammonia levels are 0ppm. I will definitely be investing in a master test kit. I'd never gotten one prior because I've never had issues like this before. The nitrates never set off an alarm because it was just the 2 fish in there.

And I had plants soon after the tank water cleared and got to temperature. I also had 3 mystery snails and was putting algae wafers in the tank for the snails before any fish were added. I also put a used filter cartridge from my established 20 gallon in the filter.
I have also used a variety of seachem products in all of my tanks over the years and have never had issues before. I'll keep your warning in mind. The fish, snails and now cherry shrimp all seem to be doing fine now.

 

[RVARach]

2 platies in a planted 36g, no nitrates are not suspicious... but then the strips aren't great, you need a (liquid) ammonia test as well. From this information, it can't be deterimined the tank is cycled or not.
Tell us more about the dates you added the fish, their numbers, and when you found them dead.
What kind of fish were the 'algae eaters'?
Tank looks nice. But you have to get rid of the bubbler bar/air pump, you can't have that together with CO2.
Is by any chance the air pump on a timer and goes off at night?



So it's not only a fake product, but one that is detrimental to fish as well? That seems pretty out there.

I know it's not of much help, but I honestly can't remember the dates of all the fish. My platies seem to be doing just fine now and I have 4 cherry shrimp in there now too and everyone seems happy so hopefully everything has properly cycled and everything is good again.
I removed the bubbler bar a few weeks ago when the pH had risen. This is my first co2 tank and I just didn't even think to take it out once I got the system up and running. Thank you for the helpful tips.

 

[the loach]

So the tank is cycled. How many fish in total did you add? All from the same store?
What is the pH now?

I have never used Stability, have no opinion about it but Armin should have told you he hasn't used it either and is just recommending another brand, it might not be the most effective nitrifying bacteria product but I very much doubt it being detrimental to fish when I look at the Amazon reviews: https://www.amazon.com/Seachem-Stability-Fish-Tank-Stabilizer/product-reviews/B0002APIIW

 

[TwoTankAmin]

Here is why I said Stability can do harm. Bacteria in a tank that cannot be supported by the water chemistry will die. Organic matter dying leads to the use of oxygen and the creation of ammonia. We rarely think about oxygen levels or what might be using it, but all of the nitrifying bacteria require oxygen. Also, the primary cause of a cloudy tank are bacteria that are able to multiply rapidly need "organic food" for this. And they also use oxygen. A number of things we typically add to our tanks can also reduce oxygen, think dechlor or Flourish Excel for example. Anything that lowers oxygen levels in a tank has the potential to do harm, but that does not mean it always will. It is a question of what and how much.

There is a difference in the levels ammonia will reach at the peak of a cycle and where it will be in an established properly stocked tank. Ammonia levels in an established tank are not measurable on our hobby kits. That is because the ammonia is being consumed as quickly as it is produced.

The thing about the microorganisms which convert ammonia to nitrate, is they are ones which can not only survive but also thrive in low levels of ammonia and nitrite. This is why in 1996 when Dr. Hovanec did the research and wrote the paper which showed that the specific bacteria strains that were assumed to be in aquariums were not. What was expected were the ones found in waste water treatment which involve much higher levels of ammonia etc. than ever occurs in tanks.

The bacteria which thrive on low levels cannot thrive in much higher ones and vice versa. Moreover the nitrifying bacteria (and Archaea) found in tanks reproduce by division. They do not form spores. This is fact. The contents of a bottle of Stability are spores. When one begins a cycle, ammonia levels spike. At somewhat higher levels the spores in Stability hatch and the bacteria find sufficient ammonia (and soon nitrite) to work for a bit, but as they lower the levels, they start to die back and form more spores. At this stage the nitrifyers we want begin to multiply. By the time a tank is cycled and stocked, there will be no sign of the Stability hatched bacteria which need ammonia or nitrite at much higher than tank levels. What there will be are the specific bacteria identified by Dr. Hovanec et al. over the 6 years after his thesis was published in a peer reviewed journal.

So, as hobbyists in need of help with a cycle, given the choice between adding what will be there for years to come or that will have died off in a few weeks. which would you choose? I have cycled well over 100 tanks/filters using a jump start from other tanks and/or using Dr. Tim's One and Only. I did my first fish-in cycle in Feb. 2001. My next one and every one since has been fishless.