High Evaporation

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Aquarium Advice Activist
Apr 5, 2004
New York
What is the normal daily evaporation amounts when using a 150 watt Metal Halide light? I've been losing about a half gallon every day! Is this normal? And should I be topping off with salt water or fresh (RO/DI only) water?

The bulb is about 6-6.5 inches from the surface of the water, and the hood of the aquarium has been removed (as the light is a clip on). The light is on timer and operates 9 hours each day. I also have a double-action cooling fan blowing between the light and water surface.
For a 45 gal tank 1/2 a gal a day is about as close to normal as it gets with or without the light.

During the winter with little humidity you will have a little more evaporation then the summer.

Also since you probably have more water circulation then normal because it's a hex tank it will also cause a little more evaporation.

Salt doesn't evaporate so FW is added daily to keep your salinity from changing too much.

SW is added at the same ph/temp/sg of the main when you take out SW for pwc.
Thanks for calming my nerves. I've always had a cover on the aquarium before, and never experienced this much evaporation.

Would it be OK to cut back on my water changes, now that I'm topping off with about a gallon every 2 days? Before, I used to do about a 20-25% change every 2 weeks. Or should I continue as before?
If your parameters stay in check, you could probably go every 3-4 weeks, but I would stay on the testing to make sure.
Keeping your tank uncovered is more beneficial and keeps from trapping CO2 which lowers your oxygen levels and also affects your ph.

As Scott said it depends more on your parameters. Even though your water is evaporating the nh3 which eventually gets converted to no3 does not evaporate and topping off daily does nothing to lower no3.

No3 can be converted to nitrogen gas which does escape your tank through denitrification.

Or macro algae pruned and kept in a lighted fuge will help export no3.

pwc also replenish trace elements which help keep your other parameters in line like alk/ph/ca/mg for example so 10%-20% once a month is still a good idea even if your nh3/no2/no3 is 0 ppm.
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