If you don't have anyway to test the nitrite, I *believe* there's a possibility your tanks not cycled yet. Ammonia sky rockets, than falls as the nitrite levels climb. Then the nitrite levels fall to Oppm as they are converted to nitrates. What type of test are you using? The stick dip ones are often inaccurate, and only 4 weeks is a short cycle, normally it's 6-8 weeks unless you seeded with filter media from another tank. In regards to your stocking though, gold and opaline gouramis are way too big for your aquarium. Adult size for gold is ~4" and for opaline ~5." They should be in something like a 3ft 30 gallon minimum. They also will get more aggressive with each other as they mature, especially if you have more than one male. Even females will fight when crammed together. You didnt get an oscar too, right? These tank busters will reach easily a FOOT long in a year, and a 55 gallon for a single fish alone is the bare minimum. They also are best kept alone, or in aquariums of 75 gal+ if you want friends.