You have several questions. Let me go through them one at a time and hopefully add to the answers you have received already.
First off,I suggest you get back in touch with Labman,seems, accidentally, all he did on his first post was copy paste your questions, and he may not realize he did not include any answers. I am sure he has some good info he wanted to share, but somehow msitakenly did not include his comments in the post.
Electricians usually offer inexpensive solutions because home owers ask for the less expensive installation. And a good electrician will know how to do a safe installation without adding unnecessary cost. A real good electrician will listen to the customer and offer a better installation, of course for more money, but in an effort to get the customer what he really needs.
Seems you are interested in a better than standard installation due to the purpose of the room. Flickits suggestions about the GFI breaker and at the outlets is a four star installation, offering protection at both locations, and Labman's suggetion of limiting protection at each outlet wll be easier to trace problems.
What I have to offer is, code requires GFI outlets in specfic areas of the home susceptible to shock hazard during general use by anyone living in the home, sinks, kitchens, basements, garages, and outdoors. However, this room may be under your control, for your use, and is for a specific purpose, aquariums. What I am asking you to consider is , how many tanks, and how many fish. If a GFI tripping may cause the tank pump to be down for an extended period of time, is it possible to loose any fish?
Even thou the tanks contain water, GFI protection of lights and pumps in aquariums is not required by code. If you are concerned for your safety, I certainly agree with GFI, but, if there is any chance you may loose rare or expensive fish, GFI is not recommended or required. This is your call. You can shut off the breaker when you need to clean the tanks to keep yourself safe, but have no fear of loosing fish if a GFI trips and your not around for a while to catch it and find fish floating at the top of the tank.
Like Flickit says, no code or permits? I sure would like to know where in the USA codes are not required. Maybe you just asked the wrong person about codes required in your town.
You are correct, a 15 amp circuit can handle 1440 watts continuously, which means more than three hours, and a 20 amp can actually handle 1920 watts. Your design of 12 outlets for 1200 watts is perfect. There is no maximum number of outets on a residential circuit, but you seem to undertand, it is not the amount of outlets, but the total load a circuit can handle.
Without knowing more detail on the heater, one advantage to having 3-20 amp circuits, is that if you have a problem with one, at least the other two are still operating. There should be no reason you need to hard wire the unit if you can use 3 20 amp cords, no longer than six feet, and have each plug into its own circuit and outlet. And the plugs can serve as disconnects at the units.
I hope I have added some information to help you.