This might not be what you want to hear but . . .
Fish tank as a guinea cage not a good idea. (!!!!!)
I have personal expericance. When we kept our guinea pigs in a fish tank they lived for 3 years ( that was the longest one).
After the 5th guniea pig died we finally went and asked our vet why our guinea pigs kept dieing and he told us that it was becasue bacteria and the kind of stuff would build up in the air of the aquarium because there was little ventalation ( we removed the old bedding and placed new beeding once a week with the acational wash out once every 6 months). Once we changed to the wire cage my last guiniea pig live to 7 1/2 years old.
I'm not sure how big the aquarium is but I would advise to get 2 unless you plan on playing with him or her a lot because they ar group animals and they need interactivity or they get lonley.
And note they do grow bigger than when you first buy them at the pet store.
Summary of my suggestions:
~Get a wire cage with a plastic bottom ( no hole is bottom)
~Get 2 guiniea pigs (females they don't fight as much as males) if your not planning to play with it At least 1 hour a day.
but also base it on the size of a cage
Other suggestions
: Don't get a plasic bowel all the guinea pig I've had have dumped it over and it wastes food.
: put vitimium C in thier water to help fight off and protect aginst desiseases
: Try not to get woodchip bedding some guinea pigs are allergic to it and it can cause feet problem long term,
: If you do use a fish tank THROUGHLY Clean it at least once a month ( like completely disinfect it) and let it air out for a hour or so outside.
If you plan on keeping them outside or in the garage bring them inside during hot days and during winter
Good and tasty treats: Carrots and apple!! They love them! not to much or it could give them dierria.
Good luck with your new guinea pig or pigs and look out for signs of illness when buying ( watery eyes, dierria or constant squinting or if one of the other guiniea pigs have these symtoms in the cage because if one has it there is a good chance they all have it. So if there is any sign of them being unhealthy don't buy any of them.)
Ps.Good luck and try looking at some books for info
Pss. Sorry about any mis spellings