unfortunately, a 3.5 gallon tank is far too small for more than a betta, and perhaps a few shrimp. 1-2 ppm of ammonia is EXTREMELY high, and quite poisonous, and would probably be the reason the betta died. Frogs produce a LOT of ammonia for such little bodies- mine lives with one snail in a 5 gallon tank. add another fish to that, and you've got them swimming in their own poison for sure.
Another part of your problem was probably that the tank was far from cycled, which you can see because your ammonia levels were not 0. If a small tank like that cycles with one fish, it would be able to support one fish. any tank that's cycled will have to catch up with each additional fish added, which is why many people only add one school, or even just a couple of fish to their tanks at a time, until levels are all caught up.
I think the biggest thing to know, for a person fairly new to fish keeping, is that pet stores are out to make money. they're going to try and sell you what they can. even 10 gallon tanks are small for a community, and that's the smallest i would suggest for what you had up there, and i don't particularly know much about half bills. a 3.5 gallon tank would really only suit a group of dwarf shrimp, one betta, or perhaps one african dwarf frog (and i know people who don't like frogs in less than 5 gallons).
For now I would suggest leaving the frog to himself in the 3.5 gallon tank. If you're interested in a community, investing in at least 10 gallons would start to give you options for a few smallish fish (and the frog if you chose to move it). of course the bigger you go, the more options you have
as for the filter cartridges, i believe most of the beneficial (nitrifying, ammonia converting) bacteria live in your filter media. changing the filter cartridge would ruin your cycle, since most of your very necessary bacteria would be removed. if your filter cartridges started falling apart, i would place a new one in the filter, leaving the other one for at least a month, then removing the old cartridge.
Good luck! Starting out in fish keeping can be kind of stressful, and I hope you take another shot at it
Another part of your problem was probably that the tank was far from cycled, which you can see because your ammonia levels were not 0. If a small tank like that cycles with one fish, it would be able to support one fish. any tank that's cycled will have to catch up with each additional fish added, which is why many people only add one school, or even just a couple of fish to their tanks at a time, until levels are all caught up.
I think the biggest thing to know, for a person fairly new to fish keeping, is that pet stores are out to make money. they're going to try and sell you what they can. even 10 gallon tanks are small for a community, and that's the smallest i would suggest for what you had up there, and i don't particularly know much about half bills. a 3.5 gallon tank would really only suit a group of dwarf shrimp, one betta, or perhaps one african dwarf frog (and i know people who don't like frogs in less than 5 gallons).
For now I would suggest leaving the frog to himself in the 3.5 gallon tank. If you're interested in a community, investing in at least 10 gallons would start to give you options for a few smallish fish (and the frog if you chose to move it). of course the bigger you go, the more options you have
as for the filter cartridges, i believe most of the beneficial (nitrifying, ammonia converting) bacteria live in your filter media. changing the filter cartridge would ruin your cycle, since most of your very necessary bacteria would be removed. if your filter cartridges started falling apart, i would place a new one in the filter, leaving the other one for at least a month, then removing the old cartridge.
Good luck! Starting out in fish keeping can be kind of stressful, and I hope you take another shot at it
