I would be leery of putting a pair of leopard geckos in a 15 gallon aquarium for two reasons. First off, unless its a 15 gallon breeder tank (shorter and wider than a regular 15 gallon), there just isn't enough room. If they are two young ones sure, until they get full grown - no problem. These guys can live over 20 years and they just deserve more space than that.
Oh and as far as 'experts' saying two or more can live in a 10 gallon aquarium -- even big-time breeders who covet all possible floor space give more room. About the only leopard gecko book I'll take as completely accurate is "The Leopard Gecko Manual" by de Vosjoli, Viets, Tremper, and Klingenberg, (published by Advanced Vivarium Systems). It's under $10 and has SOLID information including breeding specifics. If you don't have it, it is very much worth buying.
Secondly a male leopard gecko, once he's ready to breed, will 'worry' one female a lot. He will constantly be at her and either she'll quit eating and lose weight from the stress or she'll kick his little gecko butt - either way, you'll have an unhappy unhealthy situation. Two females ... maybe ... if you keep an eye on them and make sure both eat well. If you want a group, try to have three females to one male. You'll still have to watch to be sure all eat well.
Keep asking questions, its always a good thing!
>>I have a fifteen gallon aquarium with a heat pad and all the supplies. I was wondering if it would be big enough for one breeding pair of leopard geckos. A lot of people say a 10 gallon is big enough and many of them say a 20 gallon is good. I already have the fifteen, is that big enough?
-----
Andrea A. 