I have a 400 gallon Fluval filter, which cost lots of $$ and it rated very HIGH for large turtles and big tanks! I don't understand the 'filter logic' going on here with big filter vs little filter, indoors and outdoors... I could get them each separate stock tubs, but if I'm gonna go there with a filter for each one, I might as well just dig a big freaking pond and filter that... no idea what kind of heavy duty filter I'd need for that, but what I understand is, it'll still need to be cleaned often....Bigger filter you will for sure need, then a bigger enclosure. The filter needs to have a lot more surface area for bacteria colonies to grow on. I built my filter out of a 5 gallon bucket. New filter will help, but you should really think of a new enclosure or few enclosure with each one being filtered.
Kyle