topfilling aquarium after water evaporation

[gykramer]

I have a 40 gal breeder tank, 18x36". My tank is nitrogen cycled with lots of ive plants and rocks. pH is about 7.2. Temp kepat at ±75 degrees F. I have not checked GH or kH. I prefer to keep it open with no cover. As such, I find that I lose at least a 1/4" every few days. I'd like to keep the tank full to the top so I'm trying to figure out how to add water that matches my tank's parameters. Our tap water in San Francisco is 9.2 pH and 58 degrees F. Would it be best to keep a 10-gal reservoir filled with tap water that I adjust for pH and temp (heater in the reservoir) and other parameters and refill the tank from that? Or use distilled water (I have a distiller) to which I add back minerals. Seems like a lot of work; When I was a teenage fishkeeper many decades ago, I never thought about water parameters. Now is seems to be a very important issue, and I'm taking it serioiusly. How do other people deal with the situation described?

 

[Aiken Drum]

If you don't want to alter the parameters in the aquarium you should use RO/DI or distilled water to top up after evaporation.

When water evaporates it's only the H2O that evaporates, and all the salts and disolved minerals get left behind, so these get more concentrated. When you top up with tap water you are adding more salts and disolved minerals. Compared to the parameters before the evaporation occurred your disolved solids will be a tiny bit higher. Over time these tiny bits of disolved minerals creep higher and higher, increasing your hardness, increasing pH etc. What you want to do is replace the evaporated H2O with H20, and the closest you will get to that is RO/DI or distilled. Don't remineralise the top up water, it will do the opposite of what you want.

Your water change water is a different matter though. You want that to closely match the parameters in your aquarium.

 

[Aiken Drum]

Let's say you have a 100 litre aquarium with 100 disolved minerals in it. Your disolved minerals concentration is 1.

You see 10% evaporation, so your water volume is now 90 litres, but you still have 100 disolved minerals. Your disolved minerals concentration is now 1.11.

You top up with pure H20 (RO/DI or disolved) and return the volume to 100 litres, still 100 minerals and your disolved minerals concentration returns to 1.

If you topped with 10 litres of water matching the water in the aquarium with a disolved minerals concentration of 1, you would actually be adding another 10 minerals. Your water is returned to 100 litres, but the number of minerals is now increased to 110. Your disolved minerals concentration is now 1.1. Not a very big increase, but over a year, doing that once a week that tiny 0.1 adds up, increases your total disolved solids parameter, probably increases your GH and pH.

Regular water changes as well as topping up will balance these differences to some degree, but if you are regularly topping up following evaporation use the distilled water to prevent this minerals content creep.