What if your fish is swimming sideways?

If the fish is showing no signs of distress then don't worry about it. I hope you are keeping to the rules of fishkeeping. 1 inch of fish needs a minimum of 1 gallon of water. :- Every fish tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water changed weekly. If you keep to the rules your fish stand a chance of surviving. If you don't stick to the rules I can guarantee that your fish will have constant health problems.