I've labeled your image, to help you understand what's going on.
![Labeled image](https://i.stack.imgur.com/pcLTv.jpg)
Off to the left, the grounding electrode conductor enters the box and terminates at the grounding bar. The feeder coming in the top of the disconnect has three wires, two ungrounded (hot) conductors, and a grounded (neutral) conductor. The two ungrounded (hot) conductors terminate at the disconnect, as they should. The grounded (neutral) conductor terminates at the grounding bar, as it should if this is where the service is grounded.
The feeder leaving the bottom of the panel has two ungrounded (hot) conductors, which terminate at the disconnect as expected. The grounding conductor terminates at the grounding bar, as it should. And the grounded (neutral) conductor terminates at the neutral bar. Unfortunately, since this appears to be where the service is grounded, the grounded (neutral) conductor from the lower feeder should be connected to the grounding bar.
As it's wired now, the grounded (neutral) from the lower feeder is connected to an isolated neutral bar. Which means that the grounded (neutral) conductor feeding the panel is floating, or not electrically connected to ground. Without a reference to ground, the voltage potential between either of the ungrounded (hot) conductors and the neutral can be anywhere between 0 - 240 volts.
Solution
Service grounding location
If the disconnect panel is where the service is grounded, you should move the white wire from the lower feeder to the grounding bar. Or you could bond the neutral bar to the grounding bar, using an appropriately sized conductor.
![enter image description here](https://i.stack.imgur.com/bOEaYm.jpg)
Click for larger image
Not service grounding location
If this is not where the service is grounded, you should move the bare conductor from the upper feeder to the neutral bar.
![enter image description here](https://i.stack.imgur.com/UeGnJm.jpg)
Click for larger image
Based on what you've said in comments; and because it's a 3 wire feeder and not a 4, it appears that this is where the service is grounded.
Breakers and fuses are sized to limit the fault current that can flow. Each is sized to limit the current to that which the cable connected to it can safely carry. For example a 32A ring main circuit will protect a 2x 2.5mm2 cable ring circuit. Another example will be the 6A lighting circuits. These could be a single 1mm2 cable (though 1.5mm2 seems to be more commonly used).
You aren't expected to be running everything at capacity at the same time. Just because a circuit can supply a given amount of current doesn't mean that it will be doing. As long as the current draw at any point doesn't exceed the capacity of the fuse or breaker protecting it, it won't trip.
You mention 3A and 5A "plugs". This is the current above which the fuse in the plug should blow (at some point - fuses have different "trip times" for different overcurrent ratings). The appliance supplied by these won't normally draw the current marked on the fuse. For example if you had a 1W appliance, it would be likely to have a 3A fuse (the smallest common fuse for a UK 3 pin plug), but would only actually draw around 0.004A. You could technically therefore connect around 3000 of these 1W appliances through one 13A fused plug without a problem (practically however, you might find it more of a challenge...).
The important number to be considering is the total power draw (wattage) of the appliances in use at any one time, not the total of the fuse ratings.
In relation to you aquarium, as long as you don't overload the ring main, you'll not have a problem. If you total up the wattage of all your equipment, you can run approximately 3000W through one 3 pin plug. (Note though that it's common for double sockets to be rated for less than 26A, but it's not often marked on them). Your 32A ring main is therefore capable of supplying a bit over 7000W.
In the worst case, you could always have an electrician install a dedicated circuit to supply the aquarium equipment.
Best Answer
Take out the screw completely.
Then the (usually) rubber enclosure slides off the hard plastic interior.