Okay, I know I'm supposed to do water changes and whatnot, and since I have a really low fish load in a 10g at the moment, I figure that a 25% change once every week or two would probably be adequate. Here's my dilemma, a ton of water evaporates out of my tank (almost 15% I'd say) in 2 weeks time. Should I go ahead and change out 25% of it and then top it off all the way to the top, or just fill it back up to full as I have been doing so in the past?

First of all, replacing what evaporates does not fit the criteria of a bona fide water change. I would recommend changing out 25% of the original (pre-evaporation) volume. And I think a case could be made for using distilled/RO water for topping off. After all, the mineral content of the water does not evaporate with it. Over time you'll get some seriously hard water otherwise.

Of course, the best long-term fix might be to put a glass top on the tank. I'm kind of assuming that you don't have one now, unless you keep the water at 90 F or something. I know, it doesn't look as neat and clean as open top, but it's definitely cheaper than a RO unit.

That's the weird thing, I do have a top on the tank. It's not completely glass, just over the lighting, but everywhere else is actually pretty well sealed up. I have very small openings that are just bearly big enough for the filter, heater, C02, etc... so I can't imagine why I'd be loosing so much damn water.

drain the tank down to the half way mark and then fill the tank to the top, this should equate to a 40% +/- water change considering evap.

Marcel

Do you have surface agitation? Surface agitation speed up evaporation.

I have a 10G also, I lose the same ratio of water as yours. I top off my tank daily with 600ml of water+ferts. My tank is pretty much stagnant, BTW. So if you have surface agitation, I would assume 15% for 2 weeks is a right number.

Weird, I have a 10 gallon and evaporation is neglegible, although it doesn't have a glass top, and it has surface agitation by a little Aquaclear.

I suspect heat caused by the lights could play a factor. My tank has only 20 cool Watts (the bulbs don't get too hot, can still touch them).

Just shows, each tank is different...

Hmmm, interesting. Maybe heat is very possibly a important influencing factor.

I have 2x 27W PC. The lights gets quite hot on the top.

My tanks evaporate substantially faster in the winter they do in the summer, I assume it's due the humidity changes. In my house right now with 6 tanks running, more or less topless, the humidity is 30%, that's how dry is. In the summer, even with the AC drawing out some of the moisture it runs around 70%.

Same principle as clothes hung out to dry taking longer to dry in the humid summer months despite the heat than they do in the dryer, but still hot later summer months.

Please correct me if I'm wrong, this could be way off.

The owner of my LFS a long time ago told me that in an older house the wood is dryer, so any humidity is sucked up making your house alot dryer. This would cause higher evaporation.

I feel like an idiot even saying this.

I would say your LFS is incorrect. The moisture content of the lumber in your house is more dependent on the relative humidity outside unless it's so old as to not have a proper vapor barrier. Properly constructed, you should have a vapor barrier on the interior walls to prevent interior humidity from transitioning into the stud cavities where it could condense and cause problems. If you don't, chances are you have lathe and plaster walls and the plaster would suck up any moisture before your studs would have any affect. But even then, it wouldn't have any real effect on the relative humidity in your home unless. Most houses exchange their volume of air with the outside at a given rate, which is why your house is dry in the winter and more humid in the summer. It is virtually impossible to have a hermetically sealed house and it wouldn't be a very good idea to begin with.

I used to work for a company that weather proofed houses, a fan was placed in the door with and a computer measured the pressure rise and calculated the air exchange rate, then we'd use a variety of methods to seal the house up, but never to the extent that air exchange didn't take place, in fact a couple of times we recommended adding ventilation to increase the air exchange. I forget what the ideal rate is, the info is on the web somewhere I'm sure.

BTW, do you have discus? I noticed you list a tank temp of 81 degrees.

See I was wrong! lol

Thanks for the correction!

A ten gallon tank isn't very big. Changing half the water should only take a few minutes even with the bucket and siphon method.

Lot of evaporation going on in the winter. My tanks are in the room with the woodstove and I add a half gallon to the 29g and maybe a pint or two to the 10 on a daily basis. I will admit the cats will drink out of the 29g rather than drink from a bowl a dog has touched. Just consider it a mini water change.

My house is 253 years old. It sucks up moisture. I keep a big canning pot full of water on the woodstove and still it's dry in here. There is very noticeable shrinking and swelling of wood with the seasons. I'm willing to bet that the old limestone mortar and plaster sucks up a lot of water vapor along with the old wood.