I have a relatively new 20 gallon tank that I cycled carefully....I thought. I didn't add fish until ammonia and nitrite were 0. Lots of live plants, bacteria brought over on gravel and in filter material and plants from established healthy small tanks. Added 6 danios, 5 peppered corys, a mystery snail, two amano shrimp, and one neerite snail. All seemed well until I brought home a honey gouramI who became sick, lethargic and refusing food, within 24 hours and has died. Two corys died the same day the gouramI became sick and now a couple days later another cory looks like it's on its way out. The danios seem kind of hyper, even for them, but nervously hang in one corner of the tank. One was isolating himself yesterday, but now they are all together again, just acting weird. I haden't lost a single fish on my micro tanks in a year and that's why I thought I would rtry a 20 and now I'm afraid I'm killing everything. Is it possible the gouramI brought in something? Help!