Good point Seamaiden on the lake cichlids. They're well adapted to extremely hard water and should thrive better than goldfish in a res. Tilapia are tough cichlids too, they ought to work well. Goldfish are very uncomfortable at temps above 80F, but cichlids and catfish can thrive at high temps, so they're probably better choices. But, wait. Come to think of it, (plants growing in) water culture systems can have problems with lack of O2 at higher temps, too. Maybe goldfish or koi in a cooler system would be better?
Koi are known to dig up plant roots when housed in lilyponds, so I wouldn't go with koi. I have always loved goldies, but honestly, how about going even smaller? Lots of fish can tolerate a pretty wide range of temps and conditions, Africans can do well in a system @68F, and goldies/koi can certainly tolerate warmer temps (though they do prefer higher O2 levels).
How about guppies, White Cloud Mountain minnows, mosquito fish, platys, mollies (wild/black mollies are brackish water fish, I've used them housed with seahorses to help feed the seahorses with molly babies), or swordtails? How about zebra fish? How about something that's happier in the more acidic waters of South and Central America, seeing as how most folks who run a rez run it at lower pH values? How about Asian fishes, like tiger barbs?
I wouldn't do, for obvious reasons, certain species of cichlids that might be prone to tearing up the roots trying to arrange 'their' tank the way they think it needs to be arranged.
You see why a woman like me needs a fishroom with several hundred tanks. Or a job in the trade. It's so much fun playin' with fish!
