[CrazyRu]

Ok. I have same problem. I have original tank installed in a bilge. There is not much access to aft end of the tank, and it is not easy to remove.
PO reported that he noticed oil/fuel in a bilge and installed new tank in cockpit locker, successfully blocking access to water heater, water pump and water tank and throwing boat out of balance - now I have water tank and fuel tank sitting on same side.

I removed fuel gauge/access plate from old tank and cleaned it and made storage for spare hoses and belts inside. Due to sinking (almost) accident (it is another story, everybody alive, boat is still floating) I filled the old tank with water. I didn't have time to remove water immediately and I noticed that water just staying in the tank, so there is no visible water leakage from the tank.
At same time I found leaking fuel filter housing at the engine. So I started wander if the filter was the source of a leak, and old tank is OK?

I need to do a lot of preparation to pressure test the old tank - blocking all inlets, installing plate back in a place, besides I don't have a gauge nor pump.

Can I just fill the tank with some sort of dye, some sort of water based paint and see if I get a color in a bilge?

It seems like it is most simple solution for testing old tank. If there is no dye in a bilge then tank is not leaking, right?
Do I miss something?
What dye to use?