You're not getting an accurate reading with the strips.
I'd suggest getting yourself a liquid
test kit, the
API master kit is highly recommended. I wouldn't trust anything else.
I go back to what I originally thought, they're feeling the effects of ammonia in the tank.
Every time you replace the filter media, you throw out most of the beneficial bacteria your tank needs to process the ammonia.
You only need to rinse it in used tank water when you do your weekly changes and only replace it when it's falling apart.
Keep an extra filter sponge in your tank, when you do need to replace the one in your filter, the extra one will have bacteria built up on it.
I'd start daily water changes, about 50%, until you can get accurate readings.
For the bloated tetra either fast him or give him bits of thawed frozen pea, not pureed. You can try dipping it in garlic juice, he might accept it that way.
Another thing that might help is frozen daphnia.
Use Prime as your water conditioner, it'll help with the ammonia until ypur next water change.