What is a safe number of fish to add at a time?

[Snagrio]

My 125 gallon will soon be ready for new fish (going to wait a couple weeks for plants to grow in) but my question is what's the generally recommended numbers you can add at a time for a tank that size? The species I'm planning on are all small schooling ones (tetras, cories, hatchets, ect.) and maybe some dwarf cockatoo cichlids (still unsure on the final "centerpiece" slot).

The tank itself has an established bacterial colony and just recently had a few fish readded after renovation work has been done (two bristlenose, a giant danio and Australian rainbowfish, the latter two living out their twilight years as the last of a previous tank) so it's not necessarily a new system.

In the end I'd like a dozen of each tetra (going with a mixed shoal of neons and black neons), a dozen hatchets (if I can't find marbles silvers will suffice) and at least half a dozen corydoras (probably pandas but I'm not sure). Would it be safe to add 12 fish at a time or would half that amount of better?

 

[FreshyFresh]

Assuming this tank has been setup for some time, it's bioload processing ability is going to match what you've currently got in the tank. If you double or triple the bio load all at once, the tank, your filtration, etc, may not be able to process it all. You'll need to measure for ammonia and be ready for large WCs.

Can you add some more seasoned filtration or a sock of substrate from another known healthy and established tank?

 

[Snagrio]

Assuming this tank has been setup for some time, it's bioload processing ability is going to match what you've currently got in the tank. If you double or triple the bio load all at once, the tank, your filtration, etc, may not be able to process it all. You'll need to measure for ammonia and be ready for large WCs.

Can you add some more seasoned filtration or a sock of substrate from another known healthy and established tank?

That's the tricky thing. The tank was long-established when I got it off Craiglist late last month (had a bunch of large cichlids in it that have since been rehomed) and then I did a major overhaul of the entire setup. Substrate was completely changed out (white, coarse gravel to black sand) and the filtration (two Cascade 1500 canisters) was thoroughly cleaned as I don't think the previous owner ever serviced them as everything was covered in a thick layer of sludge. And the water was so dirty that I resorted to washing everything in the sink (it's well water though so there's no chlorine or other such chemicals in it).

Due to how things went the filters were off for a couple days or so until I got everything back up and running again so I don't know how much of the colonies were destroyed. But they've been running for a couple weeks before the four fish have been added so hopefully they've recovered enough. Tested the water before the additions last night and there was no ammonia or nitrite, but not really any nitrate either so, definitely will test some more now that there's waste actively going through the system again.

I think to be on the safe side though I'll add my two HOBs that are on the 55 gallon I'm in the process of dismantling to sell (which is where the four fish came from). They've been shut off for a day but that shouldn't damage too much of the colonies.

 

[Snagrio]

Apparently scratch the HOB idea. Neither of them fit behind the new tank and there's no way I'm going to attempt to shift whatever thousand pounds the thing weighs, so I guess the course of action now is to just monitor the water parameters closely for the next couple weeks before getting any new fish.

 

[FreshyFresh]

The only beneficial bacteria that might have survived would be on the sponges or bio media used in the Cascade 1500's. I wouldn't count on any of it having survived though..

Can you use media out of one of your HOBs from your 55 in one of the cascade 1500's? How about a sock or bio bag of gravel out of the 55?

 

[Snagrio]

I really don't want to mess with the canisters for a while to be quite honest. To put it bluntly, this whole project has been filled with mishaps and it's been extremely testing to my mother's patience (including getting water all over the floor at one point due to messing up unscrewing the piping on one of the filters). So I'm trying to remain in her good graces as much as possible at this point.

There's only four fish in there now plus some live plants (with more on the way in a couple days) so I'll just wait things out and have the cycle rejuvenate itself naturally.

 

[fishorama]

I think you should plan on "cycling" the beneficial bacteria with a low dose of ammonia, say 1 ppm. The plants should be fine with it & then you can add more than a couple fish at a time. You'll need to test often & be ready to do a water change (without spilling all over, it a knack, lol). Practice makes less messy & an old towel helps too. You want Mamma to like your tank!

So what fish did you keep? Sorry, I forgot you have fish now, I thought you rehomed them...So DON'T dose ammonia...

 

[TwoTankAmin]

It is almost impossible to give you an accurate answer to your question. There are to many variables that cannot be accurately evaluated. I will offer a bunch of bullet points as to why.

- There is a decent amount of the bacteria living in the substrate of most tanks. So changing it probably removed that bacteria.
- A very dirty filter loses bacteria. This is due to flow reduction through the media. Big cleaning also removes some amount of bacteria.
- Plants have bacteria on them when they arrive. But I have too few details on when and how many came in.
- Ammonia in water turn into two forms, NH3, which is highly toxic and is the smaller part. It is consumed by the bacteria. Most of the ammonia will turn into ammonium, NH4, which is way less toxic and is consumed by the plants.
- Ammonia processed by bacteria results in nitrate. Ammonium used by plants does not.
- When there is not enough ammonia in the water for the plants, they can use nitrate, albeit less efficiently than ammonia.

So here is what I can say for sure about your situation. Whatever amount of bacteria you might have combined with the plants is using up any ammonia being produced in the tank. Once the plants are established and showing some decent growth, you can add at least as many fish as you already have as long as they are of a similar size. If you are talking lots of plants and tetra sized fish, you can probably add 4 to 8 fish. I would test for ammonia the first day or two to be sure. But I doubt you will have an issue.

After that I would consider adding about 1/3 of the mass that the tank already holds for each addition. You can add fewer at a time as well, but not more. If I understand your tank plan. most of the ammonia will be used by your plants and you will have only a minimal amount of bacteria. Plants can use ammonium way faster than the bacteria can use ammonia.

No matter how many plants one has in a tank, there will still be some amount of bacteria present consuming some amount of the total ammonia (NH3 + NH4) in a tank. On the other hand one can have a tank with no plants and the bacteria will do all the work. The bacteria multiply slowly for a microorganism. Under optimal conditions the ammonia ones will do it an about 8 hours and the nitrite ones in about 11. So if you add a few too many fish such that the ammonia produced is greater than the plants and exisiting bacteria can handle, the bacteria will multiply to handle the difference.

If you do not increase the bio-load in the tank by more than 25% - 33%, you should not overwhelm anything for very long, if at all.

 

[Snagrio]

I think you should plan on "cycling" the beneficial bacteria with a low dose of ammonia, say 1 ppm. The plants should be fine with it & then you can add more than a couple fish at a time. You'll need to test often & be ready to do a water change (without spilling all over, it a knack, lol). Practice makes less messy & an old towel helps too. You want Mamma to like your tank!

So what fish did you keep? Sorry, I forgot you have fish now, I thought you rehomed them...So DON'T dose ammonia...

Two bristlenose plecos, a giant danio and an Australian rainbowfish. The former two were part of the plan as I'm going for all South American species, but the others I decided to keep due to being in their twilight years (they're the last of their respective schools).

It is almost impossible to give you an accurate answer to your question. There are to many variables that cannot be accurately evaluated. I will offer a bunch of bullet points as to why.

- There is a decent amount of the bacteria living in the substrate of most tanks. So changing it probably removed that bacteria.
- A very dirty filter loses bacteria. This is due to flow reduction through the media. Big cleaning also removes some amount of bacteria.
- Plants have bacteria on them when they arrive. But I have too few details on when and how many came in.
- Ammonia in water turn into two forms, NH3, which is highly toxic and is the smaller part. It is consumed by the bacteria. Most of the ammonia will turn into ammonium, NH4, which is way less toxic and is consumed by the plants.
- Ammonia processed by bacteria results in nitrate. Ammonium used by plants does not.
- When there is not enough ammonia in the water for the plants, they can use nitrate, albeit less efficiently than ammonia.

So here is what I can say for sure about your situation. Whatever amount of bacteria you might have combined with the plants is using up any ammonia being produced in the tank. Once the plants are established and showing some decent growth, you can add at least as many fish as you already have as long as they are of a similar size. If you are talking lots of plants and tetra sized fish, you can probably add 4 to 8 fish. I would test for ammonia the first day or two to be sure. But I doubt you will have an issue.

After that I would consider adding about 1/3 of the mass that the tank already holds for each addition. You can add fewer at a time as well, but not more. If I understand your tank plan. most of the ammonia will be used by your plants and you will have only a minimal amount of bacteria. Plants can use ammonium way faster than the bacteria can use ammonia.

No matter how many plants one has in a tank, there will still be some amount of bacteria present consuming some amount of the total ammonia (NH3 + NH4) in a tank. On the other hand one can have a tank with no plants and the bacteria will do all the work. The bacteria multiply slowly for a microorganism. Under optimal conditions the ammonia ones will do it an about 8 hours and the nitrite ones in about 11. So if you add a few too many fish such that the ammonia produced is greater than the plants and exisiting bacteria can handle, the bacteria will multiply to handle the difference.

If you do not increase the bio-load in the tank by more than 25% - 33%, you should not overwhelm anything for very long, if at all.

That's the difficult thing. Some folks on another forum were even telling me that, once everything's fully established, that it'd be better to add almost if not everything at once. It'd be nice to do that as I could make an online bulk order but surely adding even half the amount of fish I want in total in one go would likely lead to disaster.

But on the other hand making several batch introductions increases the chance of disease spread and due to circumstances I don't think I'll be able to quarantine (will only have a 10 gallon available at that point as the 55 gallon must be sold off and I won't have any bacteria-rich filters to pull from another tank). That and my mother probably wouldn't be happy about another tank running even if it's temporary.

Believe me, if I had free reign of choice the 55 gallon would still be running and the 125 would be allowed a full month to cycle and plants grow in undisturbed with no fish around. But I don't live on my own yet so I have to abide by my parent's wishes, who are already being gracious to allow this expansion already.

At the very least however, I most definitely won't be hasty with adding any more fish for a while by default. Both for the sake of letting the cycle fix itself and so the plants have a chance to grow in however they can. Not to mention if I do decide to go the online bulk route, it's too cold where I live right now to ship anything safely anyway.