Abbie...
I always use a fluval filter in the main tank in the lounge, however in the breeding tank I have a fluval 4 plus with a filter on set to maximum (this filter does have tight around), this same filter does not pick up as much waste as it used to as the tights keep some waste back
( though I use it as I think they are worth their weight in gold......
and also a fluval 3 plus in the same tank without stocking, all I do with that filter is drop the adjuster to half way between
max and minimum, as such you should have less fry pulled up into the inlet and hopefully less untimely deaths...
it is inevitable you will get a few losses at some point.....
I am working on a tight budget right now being out of work... but would recommend swapping to sponge filters if you can afford it...
or just add one alongside until the new sponge media is fully loaden with good bacteria....
since i HAVE ALOT OF BRISTLES.. breeding out in the tank right now I think I am always going to have two filters set up in my 26 gallon tanks at least.....
my local LFS sells the sponges at £2.50 to £10 depending on the size your after so you should be able to get some fairly cheap....