Mice make lovely pets but if you are planning on keeping them in a fish tank you would need to make sure the lid is well ventilated and even then you would need to make sure they don't get too hot in summer.
Mice are a reasonably cheap pet to keep but the females can be prone to tumors and may need veterinary care later on in life. The females can live happily in pairs or groups but only a single male may be kept because two males will generally fight to the death.
Apart from keeping their enclosure clean, providing them with hiding places/houses and food and water they are pretty low maintenance and a lot of fun and very sweet. If you have any specific questions don't hesitate to ask. I am a mouse lover and had 13 plus a rat not so long ago and I know there are a lot of rodent lovers on here which I am sure would only to to happy to offer advice