Fluval 106

I have just found what I was looking for in the 106 manual. On page 11 here it says

Adjusting Water Flow
Once the filter is operating continuously, you can use the top lever on the AquaStop to regulate the water flow.
To reduce the water flow, lift the lever up to halfway, but no higher. This can be done once the unit is fully operational,
and will not harm the motor. However, never run the filter with the lever up (closed) more than half way.
Air

This would make it suitable for a 64 litre tank.

The instructions say it comes with all the media - I would replace the carbon with crushed coral.

thanks guys for your reply, I'm going to get one and hopefully will be ok, will the crushed coral stabilise my ph sue,

will do , thanks :-)

The U series has adjustable flow, doesn't it? If so, you could turn the flow rate down if it proves a problem for the fish.

Do you intend moving media over or just running the two together for a couple of weeks?

Unless you transfer the media, you will need to run both for 2 months to get the bacteria to grow in any significant numbers in the new filter. If you don't want to use the old media, removing it bit by bit over a couple of months till it's all gone will get the bacteria to grow in the new filter faster, especially if you can transfer a bit of old media across to seed the new filter - the bacteria grow better in new media which is in direct physical contact with some old media.

The U series internals are different from externals. The water flows first through 2 boxes containing sponge (one on each side), then through 2 poly-carbon pads (a layer of filter wool attached to another layer impregnated with carbon), and in the centre is a box of ceramic noodles.
Unlike an external, there is no container filled with carbon. It is more difficult to customise. The carbon is impregnated into a thin pad so you can't replace that with shell or coral. The usual way to customise these filters is just by replacing the poly-carbon pad with a layer of filter wool.

The best way to use coral/shell I can think of is to get a small internal filter that takes a sponge rather than a cartridge and fill that with the shell/coral in a mesh bag. That's what I do if I need to use carbon.
I did manage to find a sponge filled filter for my betta's tank - Maidenhead Aquatics own brand, the Aqua Internal 50. Most filters this size contain cartridges