The above picture was my first DIY internal power filter. It worked well and encorporated mechanical, chemical (activated carbon), and biological filtration. The power head filtered about 298 gph. The plus side with these filters is you dont have to worry about leaks like other external filters, they are cheap to make, and they do well at circulating the aquarium water. However the negatives with these internal filters is that they are messy to clean, they take up alot of room in the aquarium, the fish always bang into them, and there is very little room inside the filter to house your filtration (mechanical, chemical, and bio).
(In the beginning of the video below you can see this filter in action.)
The picture above was the second DIY internal filter I made. It was a dual powerhead filter. Both powerheads combined filtered 496 gph through 3″ PVC pipe so there was more room for the filtration media but it took up a great deal of apace in the aquarium. It worked awesome but the filter would loose flow quickly due to filter clogging so I had to change the filter media every 2 days. The best thing about using dual powerheads is that I could point one upwards for surface aggitation and I could point the other one slightly down to circulate the water and make a nice current.
(In the beginning of the video below you can see the dual powerhead filter in action.)