Well, I must say that there are a lot of ingenious solutions here. In such sterling company, my own modest rig is perhaps a bit lacking in the flair department, but I have decided to come out and post it regardless.
This is a small aquarium which was under roofing down by the fjord and I used it to test if my idea to continuously pump in water from the sea to a cold saltwater tank was feasible.
I used an aquarium I couldn't drill holes in because it wasn't mine to mess up. So i improvised this little overflow thing, which ran with reasonable instability for almost a year until I had the final tank finished.
Also, it might be noted, that the light "fitting" - if such it can be called - was on a circuit which was not properly grounded and really not fit for outdoor use. It was vitally important not to touch it too much while feeding the fish.
It nearly electrocuted me on a few occasions, and a I lost a few fish to the overflow tubes, but most of the inhabitants and all the operators survived.
Yes.. I have taken it down and removed most of the evidence....
.. and NO you shouldn't copy it - because although it worked after a fashion, it was more dangerous than successful.
Regards,
Bjorn