Its a bit small for a plec really as small ones you have to keep in a group.
The tank is really to small for livebearers as well as they are active fish.
How many fish do you have in the tank at present.
They may also completely get rid of the platy and guppy fry Which I take it is not your object since you go to the trouble of keeping a separate fry tank.
I was going to suggest shrimps, but even they might have the fry. Tbh apart from clearing off the worst algae jungle- why don't you just leave it to provide a nourishing snack for the fry- infusoria while they are little, and then the algae themselves as they grow? My 60 ltr is now a species tank for phallichthys tico, a very shy small livebearer, and as they are a bit fussy about food, I am leaving that tank to look like a small wild swamp- algae everywhere and roots going down through the whole tank. It may not be pretty, but I think it's working.
thanks for all your advice i'll think i will just have to clean the tank manually .ive got about 20 guppy fry (1 week old) and 6 platy fry(4 weeks old)