To begin with ... I highly suggest reading the article in the link below on fish in cycling. Then get to work on doing 50% water changes ... you'll need to do a couple back to back to get that ammo reading below 0.25ppm. How are you testing the water? There are two products AA member highly recommend to keep a healthy tank. API test master kit and seachem prime de-chlorinator (if you don't already use them).
I just learned about cycling but I already have fish. What now?!
... you have there are two things already against you.
1) Going with bacteria (BB) already with substrate & water. To me it's like bacteria in a bottle .. may not be the most reliable. For future reference, the best way to get bacteria quickly is to get it from an established tank either by filter media, substrate or decor.
2) Having a small tank which builds up ammonia quickly requiring more constant water changes. This is why the API test master kit and seachem prime are important. Prime lock ammo and nitrites in a less toxic form for up to 24+ hrs buying your fish some breathing room .. but BB will still be able to consume it. If you could upgrade to a larger tank ... even a 10gal, then it would be highly recommended.
However all is not lost ... don't let the loss of fish discourage you ... it's happen to many members. Testing and PWC
's is the simple solution ... don't bother with any other chemicals other than water and seachem prime.
If you can rehome the fish you have in your tank you could do a fishless cycling which would actually be easier for you ... see the link below.
The (almost) Complete Guide and FAQ to Fishless Cycling
Again ... Diligence + water Testing + PWC
's = Established tank = happy fish
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.