If your aquarium looks like it's been filled with watered-down skim milk, you could be dealing with a bacterial bloom. The water will have a distinct haze, and it may be difficult to see from one end of the tank to the other. Learning to deal with a bacterial bloom involves a little knowledge about how bacteria works.
Good and Bad Bacteria
Bacteria blooms almost always occur when a new aquarium is set up. Bacteria comes when there's too much waste for the filter to handle on its own. Ammonia builds up in the tank if there are too many fish for the aquarium, or if the aquarist is overfeeding, causing food to collect on the tank bottom and decay. But new equipment from a pet store is unused and sanitized, so there's no bacteria on the substrate, decorative items and filters. So, when a new tank is set up, there are no hungry microscopic organisms -- so-called beneficial bacteria -- to gobble up the waste products that are, to them, nutrients. Sensing a great opportunity, beneficial bacteria blooms very rapidly, causing a hazy appearance to the water.
Let Nature Take Its Course
If you're the patient type, let nature takes its course and give the tank time to clear up on its own. The haze from a bacterial bloom may look unsightly, but it's not causing any harm to the fish. What may harm your fish is unhealthy water caused by too much overfeeding, poor filtration, overcrowding or ammonia in the tank.
If you've checked to make sure the ammonia levels are down, the filter is adequate for the tank and you're not overfeeding, simply stop feeding for a few days and allow the water to clear up naturally. This could take anywhere from 5 to 15 days.
Over-the-counter treatments may help. If you go this route, you may have to replace your normal filter cartridge with fine webbing -- the kind they sell in pet stores for pond filters. Cut off enough to stuff down into your filter, then let the filter run a full cycle. This is used because the bacteria causing the bloom is much too small to be filtered out by normal filter cartridges. Products called water clarifiers attach to the bacteria, clumping them together so they're big enough to be filtered using normal cartridges. If you have ghost shrimp in your tank, read the label, however, as not all clarifying treatments are safe for them. These products often work quite well to clear up cloudy water.
Adding an extra airstone to your tank will help dissipate some of the toxins. If you only have one fish, such as a betta, set up a small tank to keep him in so you can work more efficiently in the larger tank. Adding chemicals and performing water changes will stir up a lot of debris, which is what you want to do to get the tank clean, but it's not good for the fish's gills. Add a fish anti-stress product containing aloe vera to help keep the fish strong during changes in his environment. Also, add a tablespoon of aquarium salt for each 5 gallons of water (dissolve it first in two cups of treated water). This will add electrolytes to the water and help maintain tank homeostasis.
Prevent Future Blooms
When performing water changes, don't clean everything with hot water in an effort to get rid of all the built-up slimy bacteria. Some bacteria is necessary to keep the bad bacteria in check. When the good bacteria population gets too big for the available food, it dies back, leaving a crystal-clear tank. When changing filter cartridges, it's fine to rinse off the bio-filler part with cool water, but don't overdo it.