Any Advice With This Lighting Issue?

I have done a bit of poking around the web and have come up with the following. A lot of it is guesswork because I don't know what's in the tank already and what bits you can (safely) reuse.

I suspect that the lights already in the tank are twin 18W T8 fluorescent tubes. They may be 30W but I suspect that these are too long for the tank. Anyway, I will give you some prices for both.

2 tubes, T8 18W 6500K £2.22 each or T8 30W 6500K £4.62 each
1 18W T8 ballast £21.16 or 1 30W T8 ballast £26.86
4 IP64 lamp holders with flex £8.22 each

Total is £58.48 for 18W or £68.98 for 30W.

You will need to add a bit of cable and the do the work yourself. and may a few odds and ends

Or, a better option in my opinion, go for twin 21W T5 tubes. Similar bits of kit would cost £58.64.

You could replace all the fluorescents with LEDs. See here for the sort of things on offer. Depending on what you went for then the price would be similar-ish, and the lower running costs and not having to replace the tubes every year would would mean LEDs were better value after a year.