I was just thinking it would be an interesting project. Maybe it's not worth the effort. Can't combine everything, I have fish that need to be kept separately.Use the 60 gallon "sump" as your main tank and convert the 15 to a sump. It's easier to maintain one big tank than a whole bunch of smaller tanks. Less cleaning, more stable, etc. IMO you are asking for a plumbing nightmare to try and hook these tanks to a single system. What's the benefit? Lots of risk overflowing the tanks, disease, algae, etc. affecting all tanks.