Hi all,
Red fins, swollen or brown gills and red/brown fin streaks are often a result of both elevated levels of nitrite, and the elevated ammonia levels that preceded it. As a general rule aeration and oxygenation become much more important as the bioload grows (particularly in "grow out" tanks where fish are getting bigger and the bio-load growing all the time).
If there isn't sufficient oxygen reaching the biological filter to complete the ammonia - nitrite - nitrate cycle for the volume of fish, ammonia and nitrite remain in the in the water.
As this waste product rich water is drawn across the gill surfaces the ammonia produced by the fish cannot diffuse from them, and nitrite occupies the sites on the blood's haemoglobin molecule that would ordinarily be transporting oxygen.
Amquel or Prime will bind with nitrite and ammonia, but will also utilise some of the oxygen on their own. More water changes are a good idea, as are some growing plants, but you need a filtration system that can deal with a higher bio-load. Trickle filters are very efficient at achieving increased aeration because of their thin film of water exposed to atmospheric oxygen,
There are some more details here: <
http://plecoplanet.com/?page_id=829>
cheers Darrel