Top off water in a tank with a lid

[Polypnewbie]

I have a hard time telling how much water has evaporated since I have a lid on my tank and it never seems to be noticeable. In a 20 gallon tank with a lid, is there a guess how much top off water would be needed a week? How many cups? (I know not to add saltwater as top off water).

 

[mitchrapp]

I have a 20g QT with eggcrate on top of it and thin acrylic pieces on top of the eggcrate. It's loosely covered, but there are still some gaps. I usually do not lose any noticeable water (that noticeably affects the salinity), but I do 5 gallon weekly water changes.

 

[cloak]

I would just put a piece of tape on the side of glass to indicate when the tank was at capacity. (1.026) As evaporation occurs the water level would drop. Now it's just a matter of pouring in enough freshwater to bring the water back in line with the tape. Piece of cake. GL.

You can also use the rim of your tank as an indicator too btw.

 

[Reef Frog]

Use tape or make a mark with a magic market.

No way to guess what your evaporation rate is; there are too many variables. Be sure to check your salinity with a calibrated refractometer. An inches worth of evaporation may be enough to cause big problems.