?

Just neons no other fish in a 30gal, how many can be kept?

I want to get some cherry shrimp soon but before I do I would like to know all fish that can be kept with them so I can plan what I'm going to have in my tank. I have a 30gal that I'm working with. Just list some options.

I have an old tank that was used for saltwater fish a long time ago but now I want to use set it up for a freshwater aquarium. It still has some salt residue on the tank tho and it is really hard to remove just normally with a sponge. Any way on how I can safely remove the residue without scratching the tank and not take so long, while still making it safe for me to use as soon as I get all the residue off.?

I have a 5gal tank with three guppies and a spotted Cory. They're all pretty small, so I was wondering if I really need to do 40% water changes every week for just these 4 fishes.

Usually I wouldn't ask this question on here because I can google this and most answers would be simply, no. But I know someone that has kept a betta with with TWO goldfish in a really small tank and all three lives and grew in the tank. The goldfish grew to about 5" - 6" and the betta did just fine and never died. They eventually gave them away to a person with a bigger tank. But I would like to know how this is possible.

Btw the tank was about maybe 2 Gallons at most!!!