i've just bought a new tank, and it's 39" x 16" x 19" - how many gallons is this?!? i've found lots of different calculators and can't get the same answer from any of them...!

What Brand of tank is it Alex ?

172 litres or about 45 US gal

Yeah, that should make a nice upgrade for your 4 goldfish. Are they fancies or single tails?

it's a juwel. i got it second hand for £25 because the light and filter don't work. that's fine, cos i have lights and filters! my 4 fish are all fancies, two orandas, one moor and one fantail (who i swear is now a veil tail!).

Has it got a built-in filter box (most Juwel tanks do)?

If so, and the powerhead does'nt work, you can either get a replacement one or if you're gonna use your own filters anyway you could take the filter box out (it's only siliconed in place, thats what I did with mine) It gives you more space and it's easy enough to do, but be really careful!