I've been the hobby for about two years and have had four used tanks.
16 Gallon Biocube - now sold but I still have my first cleaner and clown from that tank which was a great way to get into the hobby.
75 Gallon - drilled with 30 gallon sump underneath. Made a DIY air skimmer and learned that if your intake gets clogged and you have an ATO, your tank will overflow on vacation.
60 Gallon - Purchased this one to move the 75 gallon when we got a new house. It's very high and difficult to clean but beautiful shape. Sump is underneath and small (my aching back)
210 gallon (not drilled). Just purchased used last week. I couldn't watch while the movers took it down to my basement. It's not drilled. I'll be re-sealing it myself. See my questions about a sump inside the tank. I'm going to take my time setting this one up. Making my first DIY rock!
Anyway, thank you for this forum. It's pretty incredible.
Lou