Evaporation - What to expect?

Me again. So last night we completed filling our 65-gallon display tank and the 20-gallon sump below. We didn't FILL the sump, just got it to a level where the water between the center refugium and the third chamber (where the return pump lives) is just above the dividing wall when the pump is not running. We then turned the pump on, waited a few minutes, and drew a "fill line" at the third-chamber water level, giving us an idea of what seemed like the ideal level to fill the sump when the pump is running. Today, just about 12 hours later, the water is a full inch below that fill line. Is it more likely that evaporation is to blame, or did we not wait long enough after we turned the pump on? (How long does it take for the running pump to lower the water to its running level?)

What I'm basically asking is: How much evaporation should we expect every day? We currently have no hood/covering on the top of our tank (we ordered our LED light fixture, which should be here on Monday, and that will partially cover the top, but we will probably never have it completely covered). Does having an open top significantly increase evaporation?

Do you all top off your water every day, and how much? And do you primarily top it off with RO water, salt water, half-and-half—what's your routine?


Edited by dougandtito - December 20 2014 at 2:00pm

Salt and minerals do not evaporate. Top off with plain water, preferably RO. I don't know how much is normal for your tank but my tank evaporates more in the Winter than the Summer. I doubt there is much if any difference in evaporation between and open top or covering with a canopy. Sometimes you see tanks with a glass top which would help a little but it cuts down on gas exchange and reduces the amount of light getting inside the tank due to the glass and the condensation that forms on it. My tanks are open top but I do use a net cover to help keep fish from jumping out.

I lose about 3 gallons per day in a ~300g system. 
I have been using a Tunze auto top off for about 3 years to maintain my salinity without any issues.
It is a pricey purchases up front but defiantly a worthwhile investment that stabilizes your system.
If manual top off is your only option, the more frequent the better, i.e. morning and evening, to prevent swings in salinity & temperature.

I use a Tunze top-off too. Well worth the money IMHO.

I lose about 12-15 gallons a day in a 500 gallon system. My evaporation is high because I use fans to cool the water.

I lose a few pints of water a day, oh wait, that's just me.

I lose about 2-3 gallons per day on my 200g system. It has a canopy with lots of vents and fans. The sump is completely enclosed. I lose about a gallon a day on my 65g system, which is completwly open. I lose about half a gallon a day on my 25g open system.

I use a variety of ATO systems. One's a Kent, two are DIY, and then I have another brand. I like mine the best. Haven't used Tunze though.