I have some suggestions to improve the fluval u4 use.
Buy a sheet of poly pad foam such as aqua one poly pad. Cut a square almost the size of the top of the filter(looking down from above) and place it on top of the 3 trays before you close the main lid as there are few gaps where unfiltered water passes straight through resulting in blocked bio media, impeller damage.
Cutting and using your own thicker poly pads in place instead of the poly/carbon inserts work well.
Also sliding poly pad or course foam behind the filter mount/holder prevents small fish entering the abyss at the back of filter.
Vacuuming the sides of the filter while performing wc keeps it cleaner for longer.
That aquael above looks good may give one a try, wonder if you can get poly pad into it before the bio media
I found the (old)original fluval 4+ internal filter to be more powerful and included flow indicator, but I don't remember there being any compartment for bio media?
Both of them and every internal filter I've used has released debris when removing, I find turning them off and hoovering before removal helps.