Okay, first of all, thanks for the replies. I'm a complete noob and it really helps a lot.
So no more 100% water changes, got it. I'm also upgrading to a 50 gal tank tomorrow. It is from one of my boyfriend's friends who had it for years -- so should it be cycled even though it's been scrubbed and cleaned out? Or does that mean the cycle has to start over? Also, how do I go about setting it up? Meaning even though it is or was a cycled tank should I try to get the ammonia/nitrite/nitrate levels at a certain level before adding the fish? What maintenance does a 50 gal tank need?
Are there more compatable fish with Bettas than tetras? I was thinking of getting 3 more similar fish so I'll have 6 with the Betta. Would this be overstocked? Any better suggestions?
I was using the salt with the water changes because that's what the guy at PetCo told me I needed to do, but I will stop doing that.
The cycle in that 50g was destroyed in the cleaning. You'll have to start over. Your choice as to how to proceed with the new cycle. If I were doing it, I'd set-up, decorate, plant and stock the tank. Then change water 10/20 gallons weekly while running the filter as high as the fish can handle, the tank at 82* with aeration from an airstone or sponge filter. Stock the tank lightly and gradually.
Run the tank in normal configuration (with weekly water changes as above) and temperature while cycling your filter in a smaller tank or bucket. This method is faster and provides an excess of nitrifying bacteria should you choose to stock the tank heavily.
Oh...in a tank that size, I'd always run a sponge filter in addition to any mechanical filter you may use. Good for the bacteria and good for non-Betta inhabitants.
I have to respectfully disagree a little with hallyx on this. I would set up the tank as suggested, put the 10g filter on the 50g in addition to the 50g filter or take the filter media & put it in the filter for the 50g. I would also use everything I could substrate, decorations, plants from the 10g in the 50g. I realize 100% water change has been done but by the time the 50 is set up there should be some new BB built up & it'll help at least a little in the 50. I would test the water daily & do water changes as required, if ammonia/nitrites are >.25 or nitrates >40. The 50g won't be cycled so water changes will most likely be needed more than once a week. Just my thoughts.
No disagreement at all, Shellie. That's very good advice. Always add whatever potentially live-bacteria-bearing media, substrate or decor you may have available. That makes cycling fish-in a safe and practical alternative.
I did not infer the OP had access to an established tank, is all.