OK. I just looked at the pics from MFK. A few questions and suggestions below?
1.) Do you know the GPH rating for the external pump in the cabinet? I only ask as it may need to be equivalent to water coming into sump, 1 for 1 eyeballed.
2.) It appears there isn't an adjustment on the overflow island. So, top off water has to be right. In other words, can't have too much water on top by the rim. Otherwise, it floods the sump below. To determine, have you tried to cut off the electricity to see how much water comes down to sump? It needs to be a perfect top off amount from tank.
3.) Once you add a filter media whether it's pads or poly-fill. It should quiet the waterfall sounds. If it is sounds coming from the overflow island. Maybe add some bioballs or a bunch of legos w/holes drilled (ask your son first, lol
) so the water doesn't fall from a great distance. It should buffer the waterfall sounds.
4.) Someone here mentioned about adding a cut-off or adjust valve from the overflow. It's a great idea if you are in the middle of something and need to stop flow coming down to flood the sump, or when electrical outlets are unplugged. Also, great to adjust to match the GPH on external pump.
Hope this was not too much info. Just things that I've gone through when I did my DIY 30g sump. Told myself hundred times that I wouldn't flood and have water on my new hardwood floors. First 2 week, I flooded 2x and once only after that as going on 8 months. Good luck!
IMPORTANT NOTE: If you are adding water into the tank during a water change. KEEP IN MIND, that the tank will never be full. It just goes to the sump and into the cabinet, then on to the floor. It's very stealthy and easily forgotten. Had to do this mistake to know and remember to cutoff with the adj. ball valve from the OF Box.