I’ve just closed down my Marine tank. The secret is to have as much water volume as you possibly can, so that changes to the water chemistry don’t happen too quickly.
I had a 30″ cube in the office, with pipes running through the wall and two 72″ sump tanks in the garage.
The biggest cost isn’t the fish, it’s the electricity. If you’re going to have coral then you need a high turnover of water (big pumps) and a LOT of light (Metal Halides).
When the tank was running my electricity bill was £220 a month, now it’s less than £90. Having said that, my set up was very power hungry, as I had a closed loop in the tank as well as the sump drains and returns.
With so much water volume I had to do very little maintenance to the tank, just a 10% water change every couple of months.
I only closed it down because I will be awa a lot more with work this year.