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Today I switched from 2x internal Fluval 3 plus to an external Fluval 305. The tank is 40 uk gallons (36x15x20). I have left one of the internal filters running, which I will do for some time.

My question is will I experience a spike of any kind with the introduction of the new external filter? The tank is mature - having been running over 2 years, and the basic quality of the water before the external filter was set up was:

ammonia - 0ppm
nitrite - 0ppm
nitrate - 20ppm

If I will get a spike how long will it take to show, and is there anything I can do to help the situation.

TIA.

The reason you get spikes with new filters is because they will suck up and retain waste but don't have a bacterial colony to break it down. This results in the new filter producing ammonia until it gets its own bacterial colony.

One way I prevented this was to squeeze the existing filter media into the water and let the new filter suck it up. Instantly, the new filter is colonized and in a few days the bacteria will reproduce throughout the new media. Another option would be to purchase some bio-spira to colonize the new filter.

Keeping the temperature up will speed up the bacterial reproduction. I wouldn't expect a big spike as long as you continue to run the existing filter.

So bottom line...you want to get the bacteria from the old filter into the new. If you can swap media or squeeze it into the water, that should do the trick. The sooner the new filter gets colonized, the better.

I would actually have left all the internal filters running along with the new one, and then pull one and then the other out. The old filters will get bacteria colonized in the new filter quickly.

I had to do the same sort of thing before Christmas when my very old (brown) Fluval 4 case cracked as I was taking the head off for cleaning. Quick trip to the lfs and I came home with a new Aquaone external, quickly set it up since I was mid water change and mixed the media from the Fluval with the new stuff that came in the box. 2 months later and I haven't suffered any ammonia/nitrite spikes but I did cut back on the feeding and left the fish to browse more on the plants and algae growing in the tank for a week with lighter feedings of flake etc.

You might suffer a spike after losing half the bacteria from taking one of the internals out, but if you have a spare sponge for the one your still running I'd probably take a mature sponge out of it and mix it in with the ceramic media in the external to help colonise it a bit quicker, and keep an eye on the ammonia/nitrite levels daily as a minimum, you might see a quick spike of ammonia followed by a longer nitrite one if it is going to happen.

Cutting back on feeding will also help prevent the amount of waste going into the tank that can cause a spike while the filter matures. In most cases preventing the spikes from happening at all is better than trying to treat the after effects of one but if it does happen regular water changes with water as closely matching temp/pH etc to the stuff in your main tank is good as can be temporarily swapping the carbon in the external with zeolite.

Hope everything goes smoothly for you and all this advice isn't needed .

Thanks for the info. I will move one of the sponges in to the same compartment as the ceramic media - and keep my fingers crossed all goes well.

Close monitoring of all levels will also take place, as well as lesser feeding.

I use Hagen Cycle and it works great I don't suggest many products myself but this stuff has really helped my tanks out and I can't live without the stuff. Also it don't stink up your water like some of the other products. Just my 2 cents. MIKE