I have seen reports from people that I trust that the XP4 quality is not up to the standards that I have experienced with my smaller Rena filters. I love my XP3, and the several XP2 and XP1 filters that I have but have been warned to avoid the XP4 due to seal leak problems. As far as the size filter you need for a tank, a lot depends on the fish you are keeping and the resulting biological load. My 125 gallon tank is not heavily populated and thrives with only a single XP3 on it. Many people would tell you there is no way to run a 125 with only an XP3. My own experience doing it says they are wrong. On the other hand, if I were one of the people who stocks a tank heavily, the XP3 would indeed be a bit undersized. There is only so much room inside a filter to hold media and that is the real limit to how much bioload a filter can handle. Regardless of flows, if a filter can hold enough media to serve your biological load, the rest of the tank maintenance can be done using a simple gravel vac.
Flow ratings that are used by plant people are about water circulation, not adequate filtration. Similar flow statements by fish only people are nothing but misinformation. Fish never need a particular flow rate to remove the biological products, what they need is enough media surface to hold a big enough bacterial colony to handle the load. The pumping action merely brings the water with the contaminants to the media. Any flow rate that will get the water there before it has a chance to build up in your tank is enough. That is the reason that you do not have biochemical problems as your filter starts to plug up and reduce flow. Unless the flow stops, the fish go right on looking fine and the chemical tests show no problem. What that means is that you have enough biomedia to handle your fish load. The thumb rules about flow rates have a lot more to do with the fact that a manufacturer of a filter must choose a flow rate they are comfortable using with the amount of media their filter will hold. Whatever they choose it is often based on a desire to have a filter that removes particulate. Removing particulate makes the tank visually more pleasing. Unfortunately that is only important to a fish newbie. An experienced fish keeper recognizes that biological filtration is the real name of the game. As long as the bio-filtration is taken care of, water changes or gravel vacs will take care of any particulates.
