This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Fish are gasping at the surface, or very inactive, but there are not
visible lesions when it first starts. Their fins may be clamped. Many
fish of different species are affected, and possibly the whole tank.
If the water has been bad for a while, the fish may have finrot, or
streaks of blood in their fins.
* If fish are gasping at the surface, or have purple gills: high
ammonia or low dissolved O2 may be the problem; test ammonia,
* If the main symptom is inactivity: test nitrites, pH, dissolved
Depending on your test results, try the following:
Change enough of the water to reduce ammonia levels to 1-2 ppm
for freshwater or below 1 ppm for saltwater. If that means
changing more than a third of the water, be sure the water you
add is the same temperature, salinity, hardness and pH of the
tank water. It is also okay to do multiple smaller water
changes for a few days. Aerate, and make sure pH is at or below
7.0 for freshwater tanks. In addition to or instead of changing
water, you can also add a dose of AmQuel to give fish immediate
relief. Find out why ammonia is present and correct the
Change enough of the water to bring nitrites down to below 2
ppm (as with ammonia, if this is a lot of water, match water
parameters or do multiple water changes), add 1 tbsp/gallon
salt (not all fish may tolerate this much -- start out with 1
tsp), and add supplemental aeration. Find out why the nitrite
levels are high and correct the problem.
Change water and clean the filter. If your filter is dirty,
there is more waste material present to break down into
nitrate. Start feeding less and changing water more often.
Run an airstone. If this helps a lot, the fish probably don't
have enough oxygen in the water. Your tank may need cleaning,
fewer fish, or additional water movement at the surface from a
powerhead, airstone, or filter.
If pH is too low: make sure carbonate buffering is adequate --
at least 5dKH. In general, adding baking soda at 1 tsp. per 30
gal. raises dKH about 2 degrees. For a 10-20g tank that just
needs the pH a little higher, try about a quarter teaspoonful.
If that isn't enough, add up to a teaspoonful more. You can
scale this up to 1 tsp/30 gal for larger tanks. If the pH is
still too low and the KH is at least 5-6 dKH, clean the tank.
For long-term buffering in saltwater and alkaline freshwater
systems, add crushed coral. If pH is too high, pH down
(phosphoric acid) can be added. Don't rely on this stuff,
except in extreme situations like ammonia poisoning because it
can cause excessive algal growth. To lower pH long-term, filter
over peat, or use distilled or deionized water mixed with your