Fluval Power 3 Internal Filter?

Can you be a bit more specific about the filter please, what style is it - an internal U3 or possibly 3 plus; or a hang on back (HOB) C3?



I've looked on Hagen's website and in the UK they list only the U series for internals and nothing under HOBs (though there are out of production ranges: the plain 1, 2, 3 and 4 and the Plus series). A U3 internal is intended for tanks up to 150 litres, so it would be a bit strong for 64 litres. The U2 would be more suitable. The older 3 plus was for up to 130 litre tanks.

For the USA they do list HOB filters including a Fluval C3 power filter (though you can get it in the UK, Amazon for example).  That is listed as being intended for a 190 litre tank so if you do have the C3 HOB filter, it will be a bit powerful for a 64 litre tank.

The bacteria won't all transfer to the new filter. What will happen is that you'll have the same number of bacteria you have now but they'll be split between the 2 filters in proprtion to the amount of media in each one. If one filter has twice as much media as the other, for example, that one will have ²/₃ the total number of bacteria and the other one will have ¹/₃. Then when you take the older filter out, you will lose some of your bacteria. So if you want to run both filters for 6+ weeks, after you remove the old one give the fish less food than you usually do (to reduce the amount of ammonia they make) and test for ammonia and nitrite a couple of times a day. If you see either of them, do a water change. Once you've had zeros for a couple of days you can increase the amount of food slowly up to what you currently feed.
It would also help to seed the new filter if you swapped some media between the two filters.

The other alternative is to put all the media from the old filter into the new one, chopping up sponges if necessary, and fill any gaps with media that came with the new filter. That way you'll only lose the tiny number of bacteria on the surface of the filter body.