+1 ... Your boat looks GREAT

I dont know why on earth there would ever be any kind of connection or check valve between the fresh and waste tank. Obviously something is screwed up though. I cant find any interconnect, unless it's possibly in the vent arrangement somehow?

I'm going to hook up the water fill again and see if I can watch the waste tank fill up. Maybe I'll be able to hear the water running somewhere?

I don't think they vent together, my fresh water tank has a little vent valve right on the end of tank. It would actually vent to the bilge in my boat.

could you somehow hook up a compressor (regulated down) and listen for it?

Mine vents on the starboard side with all the other vents. I have 5 vents there, 3 fuel, 1 fresh, 1 waste. Trust me, I could tell the waste vent from the fresh

I also know from filling it last year that once the fresh tank is full, it runs out the vent on the starboard side.

The 1.5" hose that tees off to the starboard side almost has to be the vent fitting, even though I dont know how you could vent the tank from the same fitting as the supply.

Silly fountain

You can get endoscopes for your Iphone pretty cheap if you can snake it back there to look... wouldnt be a bad tool to add to the arsenal eiither

Actually have one, bringing it to the lake with me next weekend

You might try alternately filling and draining each to check for possible cross contamination. Just can't believe they could be somehow tied together.

I just don't let ANYONE **** in my Fountain. Problem solved.

But that's not the problem! It would Still be overflowing!

any chance that there is a check valve for the fresh water holding tank to overflow into the waste water tank?