Filtering Question And More.

Hello Alex and welcome
Are you doing a fishless cycle?
I'm not sure about marine versus freshwater use. The only thing I think I can answer is regarding the compartments at the back. 
I have an Aquaone 22/30 obviously much much smaller than yours but it does have three compartments.

The first has ceramic cartridges, the second has a large square sponge and the third is the one where my powerhead and heater go, or at least the heater is supposed to be in there but having broken one by moving the powerhead I now keep the heater in the tank.
I have added loads of extra filter bits ie sponge in with the ceramic cartridges, ceramic noodles plus a new filter media that I'm trying out, as well as all the originals.

I would have thought, since yours is a large tank, that the 200w, although perhaps overkill from a power point of view, would fit in your tank. If it is a thermostatic heater it will go off when it reaches the correct temp.

I'm pretty sure I read somewhere that marine lighting is different from freshwater. something to do with the spectrum but I'm sure somebody else will know. Perhaps just changing the bulbs will be fine since the fitment itself should be ok.

I don't know how much you have to clean a tank that was marine to make it suitable for freshwater. I would think as long as it's been thoroughly cleaned and rinsed it should be fine. I use baby bottle tablets and rinse well afterwards rather than use bleach.
I'm sure somebody will be able to answer more fully. Sue will know the answers to the back compartments

Marine tanks usually have different lights from tropical ones. I think it is because corals need different wavelength light from plants. With PL bulbs you won't have much choice but a big store should have a few to choose from.

Is this your tank? There is a better description and photos here and a guide to setting the tank up for marine. The main difference is that the tank originally came with a protein skimmer which isn't used for freshwater.

To be honest, if there is no pump with the tank but there is another filter, is there any way to remove the filter box at the back? That would give the fish more room and probably be easier than trying to get a pump to fit.

Strictly speaking, the heater is bigger than necessary. Some would argue that if the heater were to fail in the on position it would cook the fish that bit faster than the correct wattage. But an over sized heater won't have to work as hard as a lower rated one. Provided it turns on and off to keep the water at the temp you want, it will be OK.



Re the fishless cycle, did you add ammonia at the beginning of the week it has been running? (it's just that you didn't mention it....)

I'm most appreciative about your response with the filtration and even information with the materials, though I'm curious, you said you have a pump to push the water out, but didn't mention anything about any way to suck the water into the first compartment, will the water naturally push itself through? Or would I be missing something with a pump alone?

The water enters the internal filtration by overflow from the main body of the tank into the filtration area via the slots cut into the top of the filter box at the left hand side.