you can go with a 304 fluval the filteration is great but they don't give out to much top water circulation so its nice to have something with it as well, i put in a 12'' airstone to help out my pleco so he doesn't have to surface for air always in my 77gal. yes a 304 would work don't hesitate to buy
fluval 304 260 gph aquarium compacity 70 u.s. gal
fluval 404 340 gph aquarium campacity 100 u.s.gal
The 304 will be fine for now, since your waste burden is low (small red's). But in a short time, you may find you have trouble keeping up with the burden. I have a 304 on a 30 gal community tank and it works great. I modified the output port so that it ejects closer to the surface and not submerged. This has the effect of maintaining a lot of turbulence on the surface which enhances oxygen absorption. Just get used to the idea that when your red's get to about 4" each, they will probably generate more waste than a 304 can manage. You may be able to overcome this with more frequent water changes and careful management of the food you give (try not to overfeed). Another thing you could do is use a diatom filter occassionally. I think you can get the small one for about $125 or so. That pulls everything out, but it is extra work, let me tell you!
The bottom line is that 5 piranha in a 75 is on the tight side once they get to be adults. If a larger tank is out of the question, you may want to upgrade to a better filter, or do the extra work mentioned above. Another option is to give 2 red's away; 3 adults in a 75 would be a comfortable fit for the fish as well as the 304.