What are the DO requirements of fish?

I do not know what you mean by "DO requirements" I can however give you the basic requirements of succesfull fish keeping.

The basic rules for successfully keeping any fish are :- 1 inch of fish needs at least 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week. You then need to supply the different requirements of the species of fish you wish to keep. Feed the fish morning and evening as much as they can eat in 15 minutes and never leave uneaten food to rot in the tank. Keep to the aforementioned requirements and your fish could do well. Fail in any of the above and I can guarantee that your fish will have constant health problems and will have a shortened lifespan.