I've had koi for many years, but they're in my pond. I personally wouldn't try to keep koi in a fish tank, unless it was a very large one. I hope I don't burst your bubble, but the only special care they need is plenty of room as they grow. I have a 150 gallon pond, and it's only big enough to accomodate about 4 koi and I'll eventually have to get rid of them because my pond is too small. Luckily my next door neighbor and my dad both have large ponds and can take them if need be. You should have a back up plan for when they outgrow the tank. It probably won't be long with a 10 gallon tank.