Nitrite Problems!!! SOS

[jillinsavannah]

I have a 100/120 gal aquarium with 10 small tetras, 2 chinese algae eaters, 1 small loach and a pair of angels who are about 3" diameter in body size. I run two Fluval canister filters and did have a 3rd hooked up (304 - 70gal each). The tank is half pea size gravel and half sand with glass bottles, slate, a couple of plastic plants and a piece of driftwood. For the past two weeks I have had a severe nitrite spike - 2, 5, and more!!! The lfs gave me chemicals and said quit feeding and do NOT do a water change, but it did not help. After a week of treating, everybody was at the top of the tank gasping - I did a water change of about 30%, which I think saved my fish (after losing my three elephant nose - 1 the first of the week and 2 four days later). The nitrites immediately came down to .5/I guess diluted - but now, two days later its back up to 2+!! Is there something in my tank creating the nitrite or is it just the fish or what???? I was keeping the temp at 82 degrees because I hopped to add a couple of discus, but won't consider anything until this is all well under control. Would lowering the temp help? Would adding peat pellets or something to the filters other than biomedia, charcoal, water polish fibers and the normal sponge/foam filters help? Do I need MORE chemicals?? No or very little, .25 at most, amonia - Nitrates running 40 before water change and 10 or so after. SOS!!