They are nice fish, very nice, so there is nothing wrong with wanting to keep them. If you decide to keep them, then if they are going to be the main fish in your tank, you probably would want to provide them the most ideal conditions possible. Being "tolerant" of wide varieties of conditions does not mean "thrive" in wide varieties of conditions. Since I suspect these fish were wild-caught (or else produced in a commercial fish farm), either way they probabably have spent most of their lives in their "normal" temperature range of 75°+, and the three weeks they have been in that pet store in coldwater tanks are the exception, not the rule.
This page has some very good info, and recommends 75-82° normally and the upper range of that (79° and up) for breeding.
This page says 74-82°. Here's
one breeder's account of his successfully breeding them, notice his temps were in the 78-80° range. I could go on and on. I found one or two pages that hint temps as low as 72° might be possible but in general, most recommend 75° and up.
They are basically all agreed that you ideally want soft rather than hard water, and ideally in the acidic pH range (so pH less than 7). Adults can apparently survive harder water and pH's up to about 7.5 but it sounds like there is no chance of breeding without softer, acidic water. They prefer tanks where the lighting is more subdued, with a fairly substantial amount of plant cover from both planted and floating plants. Subdued lighting and plenty of plants help to bring out their colors, and also lead to more natural behaviors.
As to how to raise temperatures, easy. You put a thermometer in the tank, set for a little bit *below* what your current temperature is. Then each day, raise the temp by about 1° per day until you have the temp where you want it. You could probably even go 2° per day, but 1° per day is being extra cautious. If you're looking for a heater, I'm partial to the
Stealth heaters as they are shatterproof and very accurate, and many aquarium shops will carry them in stock. They cost a bit more than those cheapo glass heaters but I've heard enough horror stories about glass heaters shattering that I think it is well worth the money to get a shatterproof one. (There are other brands of shatterproof heaters too, but I've personally used 4 Stealths in my lifetime and have had very positive experiences with them.)
