Yesterday, I added a sumpless ATO from XP Aqua to the tank. It’s a really clever gadget that not only keeps the water level but also skims the surface. Since we started adding Selcon to the fish food, the surface has been nasty - this thing cleared it up immediately.
Nasty surface, yup that is one of the big problems with not having an overflow. You just need something to pull that top layer of oily crap off and deal with it, often people overflow to sump which is what deals with it.