New filter into fully cycled tank

[Fox46]

I'm currently combining two tanks into one due to declining numbers in both and am working out the configuration of the existing filters. Because I have a surfeit of filters, (I used to have 3 tanks) I like to keep a spare filter running in my tank in case of emergency or the need to set up another isolation/hospital tank. I've worked out which two filters will service the remaining tank, and am planning to mothball a third filter. If I had to bring this mothballed (U4) filter back into play for whatever reason and put it in an established already cycled tank, (working alongside another fully cycled filter) roughly how long would it be before it could be considered to be cycled and a fully functioning stand alone?

 

[Essjay]

Putting a third filter in the tank won't grow more bacteria, the current numbers will slowly spread into all three filters. The amount in each filter will depend on various factors, including the amount of suitable media in each filter.
After a couple of months, the mothballed filter will be cycled, but not enough for a tank with the same amount of fish as the current tank. If the three filters hold roughly the same amount of media, each one is likely to hold enough bacteria to deal with the waste from a third of the fish currently in the tank. This is fine for a quarantine tank holding a few more fish, for example, but not to set up another tank with as many fish as you have at the moment.

 

[Fox46]

Ummm. So... leaving aside what I plan to do about combining tanks and mothballing a filter for potential future use etc.
At the moment I have a 150l tank with a U4 filter in which is designed to treat 130 - 240l of water. It's supplemented by by a U3 filter (just because it was spare) which treats 90 to 150l of water. I'd assumed that if for some reason the U4 packed up, I'd be safe enough with the already operational U3 while I sorted out the problem with the U4, though it wouldn't be an ideal long term solution as the supplementary U3 would be at the top end of its capability. But if I understand you correctly, once the U4 was not working, I wouldn't have enough bacteria in the already operational U3 to maintain the current population. So I'm probably wasting electricity by running the supplementary U3 filter, I should mothball it, and in the event the U4 fails I should just take all the media out of the U4 and put it into the mothballed U3 filter? Sorry if I'm being thick here...

 

[Ichthys]

Ummm. So... leaving aside what I plan to do about combining tanks and mothballing a filter for potential future use etc.
At the moment I have a 150l tank with a U4 filter in which is designed to treat 130 - 240l of water. It's supplemented by by a U3 filter (just because it was spare) which treats 90 to 150l of water. I'd assumed that if for some reason the U4 packed up, I'd be safe enough with the already operational U3 while I sorted out the problem with the U4, though it wouldn't be an ideal long term solution as the supplementary U3 would be at the top end of its capability. But if I understand you correctly, once the U4 was not working, I wouldn't have enough bacteria in the already operational U3 to maintain the current population. So I'm probably wasting electricity by running the supplementary U3 filter, I should mothball it, and in the event the U4 fails I should just take all the media out of the U4 and put it into the mothballed U3 filter? Sorry if I'm being thick here...

Not being thick at all. You understand it. This is exactly what you should do.

 

[Essjay]

Ummm. So... leaving aside what I plan to do about combining tanks and mothballing a filter for potential future use etc.
At the moment I have a 150l tank with a U4 filter in which is designed to treat 130 - 240l of water. It's supplemented by by a U3 filter (just because it was spare) which treats 90 to 150l of water. I'd assumed that if for some reason the U4 packed up, I'd be safe enough with the already operational U3 while I sorted out the problem with the U4, though it wouldn't be an ideal long term solution as the supplementary U3 would be at the top end of its capability. But if I understand you correctly, once the U4 was not working, I wouldn't have enough bacteria in the already operational U3 to maintain the current population. So I'm probably wasting electricity by running the supplementary U3 filter, I should mothball it, and in the event the U4 fails I should just take all the media out of the U4 and put it into the mothballed U3 filter? Sorry if I'm being thick here...

The total numbers of bacteria will be spread between the U4 and U3, but with the U4 having bigger foams and possibly more space for biomax there will be more than half the bacteria in the U4.
The U4 is big enough for your tank on its own. And if you were to run just the U4 and it were to fail, then yes, put the media in the U3 - you may need to cut a bit off the foams but losing that small amount won't harm the fish.

The 'filter' bacteria also grow on every surface in the tank, not just in the filter. So losing one filter to a malfunction doesn't mean losing half the bacteria.
You don't mention whether you have live plants in a tank - plants in a tank means less bacteria than a tank with no live plants as plants take up the ammonia made by the fish and they don't turn it into nitrite. The ammonia eaters have less ammonia and make less nitrite for the nitrite eaters so fewer bacteria grow.

 

[Fox46]

Thanks for that. I only have one small plant in the tank - and that's on its last legs as I've never had much success with them. I won't replace it when it dies. I've now effected the transfer of the 10 fish into the larger tank containing 5 fish and sorted out the filter configuration. Just one more question, as a matter of interest - the box that contains the bio max media in the now spare unused filter - would the bacteria in that stay viable if I were to just leave it in the tank, say behind an ornament?

 

[Ichthys]

Without water flowing through / past it anymore it won’t be getting as much food (ammonia, nitrite) or oxygen as it got in the filter, so most of its bacteria will die.