Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Well, I can give you a quick rundown on the most probable causes of cloudy water and you can take your pick.
1) Substrate got stirred up. Did you put a plate or plastic bag over it when you filled? If the water entering the tank stirred it up, now the filters and PH are keeping the fine matter in suspension. Answer: Fine floss in the filter, diatom filter.
2) Bacteria bloom or the beginnings of green water algae. GWA looks white-grey for the first day or two, then turns green when it is dense enough to see it as green. Heterotrophic bacteria feed on all the organic matter in the tank. Old mulm from the used substrate, skin oils that are on all the surfaces from when you set up the tank. Answer: Wait it out. Heterotrophic bacteria will eat all the stuff then die off. Usually takes a week, rarely 2 weeks. If it turns into GWA you can deal with it when you know what it is.
3) pH altering materials. Adding minerals that will alter the GH, KH and pH can make cloudy water. Answer: if you want these changes, you might need to find different materials that are compatible with your water. Some rocks can do this, too. If you think this may be the answer test a sample of each type of rock in a jar of water. Test GH, KH, pH, TDS and watch for cloudiness. Run this test for several days to a week. Answer: Remove the rocks.
4) Other: A new tank should not have these problems, but...
When my large fish really get going in one tank they kick up a lot of clouds. It settles pretty fast, unless the fish keep on making messes. Answer: more frequent maintenance, vacuum, clean the filter so it removes debris more efficiently.
Dead fish can cloud the water pretty fast. It will also smell bad. Answer (duh!) Remove dead fish. Do enough water changes to get the ammonia back down, add carbon to the filter for the smell.