Here's a pic of the setup. The water pools underneath the DIY filter but it's an old cabinet and the bottom is a little bowed downwards right there, so it could be coming from anywhere.
The filter is a Fluval 206, the output goes into a house filter with an airpump injected through a T in the PVC, and then to the tank. I haven't noticed any decrease in flow or any other problems.. I think it's probably a leaking a small amount at a time, but I haven't been able to find it.