Long post..but safety first.
GFCIs work by sensing the outgoing power hot line and comparing it to return neutral. If there's an imbalance of 5 milliamps it will trip (100 mA can kill a person under the right conditions so GFCI is to protect you from that - house circuit fuses trip at 10, 15 or 20 amps long after you could be dead). So it's looking for some degradation in equipment causing unintended electrical path to ground and tripping (ground fault). This prevents you from becoming the path to ground...you never want to be the path to ground.
It is not like a house breaker/fuse. That will trip on total current (and that's designed to protect house, not you). If your house breaker/fuse isn't tripping now and it's in good condition - then it's not total current but that imbalance. You could overload the circuit and have the house breaker trip and NOT trip the GFCI (they do different things).
So causes...equipment is faulty, GFCI trip circuit is faulty or installed incorrectly, there's moisture in the GFCI box, house wiring insulation is breaking down, overload (but only because of sensitivity, not design) or perhaps cumulative effects of small grounding issues (one device aging might have small current leakage but not enough to trip, multiple devices could add up). I think that's it.
I have some knowledge of electrical equipment and test gear so I would have no issue checking for current leakage. You state you don't so I won't cover here. You hint that that you can't get at the GFCI box since it's behind your tank (that's another problem you need to address - not having easy access to the electrical outlet is not optimally safe). So you can't check if there's condensation building up in the box and you can't replace or test the GFCI easily.
So what I recommend is find someone with electrical knowledge to assist you in your home...like an electrician. They can use test equipment and find source assuming it's failing and not something intermittent.
But...I'd also say get an aquarium ground probe (increase safety as the ground probe is designed to be best path for current take instead of you). Then check the obvious...wet/damp/salt around the outlet, near your switches/plugs/controllers. If your electrics are in your cabinet consider taking them out or putting in fan to reduce humidity. Replace any older or obviously cracked/damaged equipment in water (anything past warranty). Move the power source to a different outlet that you have access to (install GFCI there). If it still trips intermittently, you're likely going to need that electrician regardless.
The GFCI is doing it's job but make sure you test it frequently even and especially after it stops tripping occasionally (assuming you can reach enough to test it - if you can't even test it, then you need to move power elsewhere now).
Side note: So there's nothing wrong electrically or functionally with everything into one GFCI outlet, it's just a higher risk if the GFCI trips that you lose all power to everything. So it's more of a failure in redundancy.
The fact that you go weeks between trips tells me that you probably don't have any defective equipment but in the "perfect storm" you are overloading the circuit. I mean all your devices are drawing current at the same time.
