Whilst air driven (sponge or box) filters are very good for fry/breeding tanks, don't forget that if you remove your current filter to replace it, you'll leave your tank uncycled.
If you wrap a piece of sponge around your current filter (it will need to be a coarse one so it doesn't impede the flow), that will stop the fry being sucked in, and you can, later, use that to 'seed' an new, less powerful filter.
That depends on how well the tank is established.
It is true that there can be a lot of beneficial bacteria (BB) in the substrate and on the decor. However, a filter is a great place for a BB colony...and if the colony is strong enough, there may be only slight amounts elsewhere.
If you were to switch to a sponge filter, I'd suggest setting it up, then 'cleaning the existing filter media right in the tank so the new filter will receive a good dose of BB.
Alternatively, I'd suggest a sponge pre-filter (Fluval has a decent one) for the filter intake. In the meantime some mesh or even a piece of nylon panty hose will do the deed.
We don't know what kind of filter he has. In a previous thread he mentioned living in Norway, and I have no idea what kind of filters are sold there.
For example, when my shrimps were in their own tank I used an Eheim Aquaball filter and there is no way a prefilter could be attached to that. I used a sock made of the same stuff as tights (pantyhose) pulled up over all the intake vents and that worked well for the aquaball.
Yes, I live in Norway. I use Eheim internal sponge filters. External filters are available here, and verious internal sponge or sponge/charcoal filters. I haven't seen box filters. The only pre-filters I've seen are for external filter systems.
I use a sponge filter powered by an air pump in my betta's tank as the flow is not strong. These are also ideal for fry. This is the one I have, and an Eheim air pump which is much quieter than the JBL pump I had first. It comes in various sizes.
This sponge filter was recommended by the man at the shop because it has a weighted base and he'd heard of the kind that attach to the glass with suckers becoming unstuck and floating where they don't work properly.
Is there anything like this in Norway?