Here’s my advice for new reefers, buy everything like you’re gonna eventually upgrade to a bigger tank.. Let’s say you have a 75 gallon but are craving a bigger aquarium such as the 125. If you know for sure that you’re gonna end up with a bigger tank eventually then instead of buying an fx4 filter you should get an fx6.. There are lots of things you purchase for a larger tank that you can use on a smaller tank. Powerheads, uv filters, thermostats.. etc