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How long will it take for an established tank to lose it's nitrifying bacteria if there are no fish in the tank? Weeks? Months? I'm sure there are variables but let's assume a well maintained aquarium without a bunch of decaying matter.

I'm up and running with my new 75 but I've decided to keep my 29 setup for quarantine, feeders, etc. I figure I could always leave a few feeders and a snail in there. What about putting food in it as some people suggest for a fishless cycle?

If it is already established, the bacteria should die back and eat dead bacteria for a short time, and then it will level out dependent on the food source. You could feed it (it gets messy), but you might be better off just snagging the filter and popping it on your 75 until it is needed, and just running something small on the 29 to keep the water moving.

You could keep feeders in there, that would work, but you would need to have something to feed them to if you needed the tank available. Also, you'd be a bit limited given that the biofilter is at least somewhat dependent on the waste output, so if you had 4 resident feeders and then you bought a bag of 30 or so, the whole thing could spike/cycle anyway as it would be unprepared for that amount of waste.

For animals, the entire universe has been neatly divided into things to (a) mate with, (b) eat, (c) run away from, and (d) rocks. - Terry Pratchett

If there is literally no organic matter, a few days at the most. If you're just scooping out the fish and leaving all the mulm and whatnot in there, you'll probably keep a decent sized colony for a couple of weeks.

The feeders would work, but I'm not sure what you would do with them when you decide to use the tank or (if you're using goldies) when they get too big for the tank. Honestly, the easiest thing to do is probably just get a decent-sized sponge filter or even a smallish power filter and run it in your main tank. When you need to use it in the 29, just transfer it. That's probably easier than trying to keep the 29 fishless-cycled indefinitely.

If you do decide to try to keep it fishless cycled, get your hands on some pure ammonia (well, it won't be PURE ammonia, but just some clear ammonia cleaner with no scents or anything other than ammonia and water in it) and dose that to keep it going.

How bout you get a few platys or swordtails for the QT, so that their fry will provide food for them and the newcoming fish. They wont get too big for the 29, and i am sure that they will sustain a nice size of a colony. I would think that a lone apple snail could sustain a nice colony size, if fed accordingly. For a fishless cycle, use gravel from previous tanks to jumpstart it, along with a bacteria starter, and probably some ammonia like joe suggested.

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I was mostly just curious about the time frame. I like the platy / swordtail idea. It's a coldwater tank though. Maybe some guppies. I wouldn't mind limited maintenance on the 29 if it were lightly stocked.

Just had an idea. My 75 has a 4 biowheel canopy. I could snag a wheel or two to get the 29 jump started if I needed to. Too bad they are a unique size. Would be very handy if they made a HOB that used the same wheel.

Edit - Platys are cold water Maybe I'll start there.

Platies are more temperate/tropical, but they can handle unheated water, just make sure it doesn't get properly cold or else you'll risk a bundle of infections. Platies or swords would be your best choice if you want to keep it stocked.

Be careful if you are using it for quarantine though, the stress of existing inhabitants defending territories (and all livebearers mentioned are territorial to a degree) can be a bit of a problem.

For animals, the entire universe has been neatly divided into things to (a) mate with, (b) eat, (c) run away from, and (d) rocks. - Terry Pratchett