A couple years ago I "graduated" from my 20 gallon to a 65 and a 6 gallon Fluval Edge (Craigslisters). The original tank had been up and running for 5 years and I just felt it'd be a waste to toss all that well-aged bacteria, gravel, etc. And besides it seemed like a good idea to have an unoccupied tank for emergency quarantines, or other emergencies so I kept some plants and snails in it and maintain it with a heater and a cheapo Eheim HOB. It also has a bunch of swords (plants) that seem to be mostly happy. Sometimes I rinse out a sponge from the other tanks to add some nutrients which mucks it up for a spell, but that clears soon enough.
I have, though, had a running battle with hair algae which won't take a hint and leave, even with regular doses of Seachem Excel. I've considered transferring back some Siamese Algae eaters and letting them chow down, but catching them first causes too much chaos.
I've also had to lower the natural pH from 8.4 or so to 7.8 with some peat. Not sure what caused it to get so high.