New filter in established tank?

Question

I've got a 20 gallon community tank with a 20g rated Penguin filter that's been up and running for over a year. The tank is currently a little overstocked, so I'm moving my threadfin rainbows and the panda corys to a 10g. I bought a new filter for the 20g, which I've been meaning to do for a while since even though my tank is doing great and is well established, I know it's better to have a filter rated for more gallons than your tank actually is. Both the old filter and the new are currently running in the 20g.

I wanted to use my old filter for the new 10g and keep the new, bigger filter on the 20g, so my question is this: How long should I leave both filters running in the 20g before the new filter has a good crop of bacteria? How much will I upset the balance of the 20g by taking out the old filter, and is it worth it? And if/when I move the old filter to the 10g with the fish, should I expect a mini-cycle or will I get a smooth transition? Because if so, I'll be moving the white clouds over first to get things established before the threadfins and pandas move there.

Answer 1

all you can do is use your discretion. when you can see enough bacteria on the new bio media then it should be good. there shouldn't any mini cycles if there is enough bacteria. wait at least a week before transferring.

Answer 2

If you let the new filter run about a week you should be fine.  what i do too is when I get ready to start the new tank is take water from the current tank this will help cycle faster and will cause less stress for the fish.  so I would take about five gallons of water from your twenty gallon tank and put it in the new ten gallon tank and fill the rest of the way with fresh water.  it also helps if you can put some of the old gravel in there too as this also contains bacteria.  if you do move gravel your tanks will be cloudy at first but should clear up over night.

Answer 3

I'd let the new filter run for a week or so before I moved ithe old one out.  You should have enough bacteria in your tank to be fine on the old tank.  Move the old filter to the new tank and make sure you add some fish food to keep the bacteria fed.  It should cycle fairly quickly because you'll have the filter media already full of bacteria.

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Answer 4

I think the easiest way would be to use the old filter media out of the old filter in the new filter in the 20g, that's if they use the same media (I don't know the specifics). Or if you can use any part of the old filter media (sponges, etc.) in the new filter. Keep in mind that a lot of the good bacteria is actually in the gravel of the tank so the impact shouldn't be too too big.

Your new tank however will need to be fully cycled but the good news is you have the old tank to use the (gravel, filter media, etc.) to help "seed" the cycling process and cut down on the time. Good luck.