How To Best Decommission Double Filtration?

[Gvilleguy]

I recently converted my tank to house Apistogramma Cockatoos, and I want to calm the current a bit for them. I have two cycled filters running that are a few years old. So I want to decommission the HOB filter and just run with my Fluval 206 canister, which is more than enough to handle the tank. Most of the currents are being generated by the HOB right now.
 
The 26 gallon/98 litre tank bio-load is at its lowest level in a while:
2 x apisto
5 x neon tetra
2 x glofish
2 x rasbora
4 x endlers
 
So, two questions:
1. Since the colonies of bacteria are spread between both filters, I was thinking of removing a percentage of filter media from the HOB each week until it is gone, giving the canister, hopefully, time to build up some new bacteria to replace the missing ones. Would this work? Suggestions on how much to eliminate each week? Maybe 20%?
 
2. This will leave nothing breaking the surface tension of the tank - should I be concerned with oxygen exchange? I could put an air stone near the surface with enough bubbles to break the surface up.

 

[Livewire88]

Depending on the size of the HOB could you not fit all the media from it into your external filter? 

 

[Gvilleguy]

No - I have both filters completely full with sponge and bio-media. I would have to discard media from the canister to fit more in!

 

[Mikey1]

i dont see the harm in removing the HOB filter, more bacteria will develop quickly in the fluval,
 
think about it this way, if you were only using the fluval to begin with, when you replace your media in it you are removing 25-50% of the bacteria, so basically its the same thing,

 

[Livewire88]

Well I don't think you any choice other than just removing the HOB, just keep an eye on your water stats and if necessary do some large water changes whilst the bacteria in the External filter builds up to the required level. 

 

[eaglesaquarium]

+1, but if you do see a small ammonia spike, it should be fine in a day or two.
 
Personally, I'd just fast the tank for a few days starting when you remove the HOB (and couple that with a quality vac of the substrate to remove any extra bits that may persist).  That should help with keeping the spike small.

 

[Messi]

Just run them both for a couple of days and the bacteria from the HOB will give the Fluval a good kickstart. You can even take the HOB out straight away but this could cause a mini cycle if your fluval doesn't seed fast enough.

 

[Gvilleguy]

Thanks for the advice! What think ye of the surface agitation? Is that a non issue, or should I bubble it?

 

[eaglesaquarium]

Unless its going to be overstocked, I wouldn't worry about it. But if it helps you sleep at night, it won't hurt anything.

 

[This Old Spouse]

I keep a HOB filter running alongside my canister so I'll always have a mature filter to run on my quarantine tank.  If you're worried about the flow you should be able to adjust it.