no. if you don't cycle a tank before putting fish in, it will cycle on its own when you do. you can't blame it on the cory, either. the fish were probably very stressed from the ammonia, which would have made them very prone to disease.
buy some bio-spira. add it at the same time as the fish, and things should go much better. pretty much an instant-cycle.
no, you cannot keep glofish and tetras in pairs. if you want these fish, you must buy them in groups of at least 6. i personally wouldn't put glofish into a 10 gallon tank, zebra danios are much too active for a tank that small. your cories also must be kept in groups.
a good stocking amount would be 6 small-bodied tetras (pristella, silvertip, neons, glowlights) and 6 cories, preferably all of the same variety. do a 30% water change once a week, and you should be good to go
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current setup:
5.5 gallon low-light planted tank
-nothing....thinking a mini-community.
for reference: my name is Julie
