20 hex tank fo fresh?

I have an Apple snail and 4 big tetras in a 10 now. im getting a 20 hex in the morning to replace the 10. any cool freshwater fish I could add or should i convert to salt like I did with my 55?

20 hex has sort of a small footprint for a salt you need more surface area for O2 exchange.

No worse than a regular ten right? I'll keep it fresh it'll b cheaper. I already have my big saltwater anyway. What should I add to it?