Evaporation and Distilled Water

[tackful]

Hi,
I'm concerned about the TDS buildup in my 29 gal tank, which loses about 1.5 gal/week due to evaporation. By my calculations, this means that every weekly water changes causes roughly a 4-5% increase in TDS and mineral content in the tank. The cummulative effect over time worries me.
Would it be advisable to periodically add a gal or two of distilled water during a water change? Thanks.

 

[jake72]

Certainly adding distill water will reduce tds; is it needed - i just don't know. I've noticed over the past two years that the tds has increased in my aquarium but i don't know if it is a concern. I guess what I am saying is i'm kind of interested if this is a long term problem or not.

 

[tackful]

Yes, I am also wondering if the buildup over months or even years might have an effect.

 

[fishorama]

You know, TDS meters are pretty cheap, like $20 including shipping last I looked on Amazon. Your tap water may change over seasons. & you may have water or rocks that add to that effect. My TDS meter died after 13-15+ years (new battery didn't fix it) & I miss being able to quickly tell what had happened to my tanks. But it was a fun & useful tool. I'll replace it eventually. At least I got a good idea of my water after I moved to CA 11 years ago. I'm sure it's different due to drought, etc but a rough idea is helpful

 

[TwoTankAmin]

Even less than that. I have two of these https://www.amazon.com/HM-Digital-T...&qid=1679289314&sprefix=HM+TDS,aps,106&sr=8-1

 

[jake72]

Certainly adding distill water will reduce tds; is it needed - i just don't know. I've noticed over the past two years that the tds has increased in my aquarium but i don't know if it is a concern. I guess what I am saying is i'm kind of interested if this is a long term problem or not.

I wanted to provide some numbers; my tap seems to drift between 110 and 130; after 4 full years the largest increase is around 160 with the lowest 140; i typically do 2 50% water changes a week (aquariums are a bit over populated); so there has been some increase but not huge. I suspect most of the increase comes from plant fertilizer. Which i sometime add and sometime forget.

 

[Rbishop]

I'd think get the meter, check your tap regularly and compare it to your tank before and after water changes. It would also be helpful to now what makes up the TDS in your tank and tap. You may be chasing a non-issue.

 

[tackful]

I'd think get the meter, check your tap regularly and compare it to your tank before and after water changes. It would also be helpful to now what makes up the TDS in your tap.

Had not idea that TDS meters were inexpensive; xpected the opposite. Thanks everyone. Will be interesting to note changes, if any, over time.

 

[fishorama]

Yeah, I'm going to say "old tank syndrome" is a real thing You want to be careful after (let's call it politely) lesser water changes after a long while. I've had it happen but not in a long, long while (since before my TDS meter, lol). When we know better we can do better! (almost a Maya Angelou quote!...look it up!)

But the thing with Old Tank Syndrome, it's a matter of too small of water changes over a long, long time. You can't just change a huge percentage of water suddenly after years of neglect or even smallish WCs (all heck can/will! break out). Go smaller, 10% or so at most every few days or a week. A TDS meter can really help you judge how much & how often is safe.

I would go with a 10% WC/week or twice /week until your new TDS meter arrives so you can know quantitatively what's going on more exactly & how to fix it...slowly, safely...GO SLOW!

 

[tackful]

My tank is about 12 years old, although it did have a complete overhaul about 8 yrs ago. I have always followed the EI approach and done weekly 50% WC's. I hope this doen't bore the less mathmatically enthusiastic, but I recently ran a calculation assuming:
1) 10 gal tank
2) 1gal (10%) weekly evaporation
3) 50% weekly WC of the post-evaporation 9 gal

After week 1, we end up with 10 1/2 gal of TDS in our 10 gal tank because:
1) Evaporation leaves 10 gal worth of TDS in 9 gal of tank water
2) removing 50% of those 9 gal leaves a ratio of 5 gal worth of TDS in 4 1/2 gal of tank water
3) we now add back 5 1/2 gal of treated tap water to bring us up to the original 10 gal, leaving us with 10 1/2 gal worth of TDS in our 10 gal tank.

After week 2, this ratio of 10 1/2 gal of TDS to 10 gal of tank water increases to 10 3/4 to 10.
After week 3, it becomes 10 7/8 to 10.
After week 4, we have 10 15/16 to 10.

As you can see, the amount of TDS in our hypothetical example of a 10 gal tank gets ever closer and closer to 11 gal worth of TDS, but always by an increase equal to half of the previous weeks increase: 10 1/2, 10 3/4, 10 7/8, 10 15/16, etc.

My conclusion, if ypu have cared to follow this far, is that the amount of TDS in our hypothetical 10 gal tank will never exceed a ration of 11/10 of TDS to original, pre-evaporation tank water. TDS may change due to other factors, of course, but I found it helpful to realize that weekly evaporation did not entail an ever increasing TDS concentration.