This is a discussion on Guupies 101... help! within the Livebearers forums, part of the Freshwater and Tropical Fish category; -->
How many guppies can i pu tin a 1 gallon tank?
Does the tank need to be cycled?
Does it HAVE to have a ...
It would be pushing it to put just one Guppy in a one gallon tank, and yes anything you put a fish in MUST be cycled but it would be easy to use filter media/gravel from an established tank for a one gallon tank. Yes you need a filter/heater as for all tropical fish. Guppies can be kept with the same sex also.
the problem i find with guppys is that if you buy females then
the vast majority of the time they will already be holding fry !
and if you buy all males then be prepared for shredded fins.
and 1 glln of water is not going to sustain many.
Females can have a batch of fry, and continue to have fry for six months even with no males. All it takes is that one initial fertilization. So if a female has fry in your tank, you can count on more fry the following month....
If you know someone that has an aquarium ask to use their old filter media or some gravel to cycle you tank. I have a tiny 2 1/2 gal. and it has an azoo canister filter I put on it from Doctors Foster and Smith I ordered this online. Look at theirr site online and type in Azoo palm Filter it works great. I think you can get endlers to put in the 1 gal. since they are tiny fish and are smaller then Guppies. But do not get any females as you will have to many babies. They are sold as feeder fish in some LFS. I was told to put those in to cycle my small tank. You have to do a 25% water change each week to keep the nitrate/nitrite/ammonia low and do not feed them much maybe once a day til the water cycles. I also put in this stuff called Cycle and doubled the dose for about a week and after that I used it once a week to maintain the water. This will get the beneficial builogical activity going in your tank.I have had no fish loss in my tiny tanks. I have a 2 1/2 and 3 gal. I use one as a qarentine area for new fish or a hospital tank and the other has some small endlers. I have a 6 gal. and 55 gal tank also. Check out Craigslist for cheap used fish tanks and suppies in the Pets section and general in your area.
Your original question has a simple answer. I would not put even 1 male Guppy in a tank that small. A female is completely out of the question because of the fry that could be expected. That tank is a good size for a few shrimp and not much more.