The fill tubes actually fit to the outside of the tank inlets and are secured by hose clamps. I had the same issue trying to drop the "OEM" pump down the tube, it just wouldn't fall all the way in. After removing the tanks, it is apparent that the inside of tank inlets were shaved down at some point, most likely by the importer when they assembled them. I took a heat gun to the inside to melt off all of the scrapings but haven't tried the drop-in pump since. Even though the tanks are a PITA to remove, I would recommend doing it for maintenance purposes at the least. Personally, aside from wanting to make improvements, I was apprehensive of drinking water from a tank that I couldn't visually inspect for cleanliness/debris!That looks GREAT. How far down are the fill tubes extending into the tank. I have some issues when I try and use the supplied water pump, it wont drop into the tank far enough to draw water. I like your water pump set up. Keep the pics coming.