I'm setting up a 125 right now. I bought the Rena XP3 after much research...most of it on this board. Sounded like a good filter for the money. I've always had good luck with Penguin products, but from what I've read, the Filstar outperforms. I was also going to go with an undergravel, but for my tank (72X18) it would have required 3 plates and I didn't want a powerhead right in the center of the back wall. Plus, again, after much research I decided, it really wasn't necessary and sounded like more of a headache than it was worth. I am also putting two Marineland Powerhead 1140's in the tank - one on each side. This should give me about 10X circulation. For now, I'm only going with one XP3 - the fish I keep are relatively small. Also, just to clarify, there is room in the XP3 for 6 different types of media in 3 baskets. It comes with a course sponge in the bottom, then a finer sponge just above that. It also comes with a pouch of activated carbon called "chem zorb". So there is room for 3 other types of your choice. I went with 2 sets of bio-media and a water polishing micro sponge at the top.