Hi Afroturf,
Regarding the flash, I'd been asking myself the same question over the last few weeks as to what flash to get for my DSLR (I was increasingly fed-up with my on-camera pop-up flash spoiling photos and burning out pictures). Unfortunately I came to the conclusion that good-quality flashes are hard to find for Canon, although there are a number of budget flashes, they might not be quite as compatible or versatile.
I have been finding myself doing more and more people shots (a succession of weddings, I think I've reached that time in life...) and I need to be be able to angle the flash to bounce it and get a softer, less direct light. I wanted a Canon flash as they should talk E-TTL 2 properly (the code that Canon cameras use to tell the flash the details of the shot being taken so it can automatically configure the flash for the correct power and area). The only flashes that Canon offers that allow these things are the 430EX Mk2 and 580EX Mk2 (which are both fairly pricey). There is one cheaper flash, the 220Ex, but this is not powerful enough and has a fixed flash, you can't angle it at all (and it's still £127!).
I was lucky and picked up a 580EX Mk1 secondhand from the wedding photographer at one of the most recent weddings I attended for £180 (they're going for about £200-£240 on eBay), this is almost the same as the Mk2 version without some additional weatherproofing and it's a little noisier (a very very slight flash-whine but I've not really noticed at all). The Mk2 version is going for about £300-£350 on eBay and £419 in Jessops! (and they wonder why they aren't getting as many customers as they used to?!)
I'm planning on getting a wireless transmitter (an ir reciever is built into the flash, I think you set the flash to "slave" and it'll fire when it recieves an IR signal from the transmitter that is connected to the camera), or I'll get an extension cable later on. This should hopefully avoid excess light from the flash and should allow me to position the flash where it'll produce a good shot of the tank without bouncing off the glass.
A good macro lens will help you no-end, look to go as low aperture-wise as possible (f2.8 probably, anything lower is stupidly expensive), the price of the lens will go up immensely but it'll allow you to capture action properly without a flash providing there's fairly good light in the tank to begin with. However with a flash, a higher aperture will be less of a handicap.
That's just my advice from what little I know, I'd wait and get a second (or third) opinion from the other guys on the forum as they know quite a lot more than me regarding aquarium photography or might know of a decent 3rd-party flash that's cheaper
.
Hope that was of some help,
Cheers,
Matt