Why do guppys swim up and down?

If you are doing everything right they do it simply because they can. The basic rules are :- 1 inch of fish needs at least 1 gallon of water (an active Guppy needs at least 3 gallons but 10 gallons is better for a couple). :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week.. A Guppy likes a pH of around 7 to 7.2 and a temperature of around 70F to 75F. They should be fed as much as they can eat in 15 minutes every morning and evening.