As long as you have the corners of the tank directly supported, 1x4 would be plenty strong.
I'm partial to using 2x3s and 2x4s, mainly because there is just a lot more room to hold screws and such (and they are pretty cheap).
I'd avoid the metal brackets. If it helps at all, try thinking to build in such a way that screws are almost unnecessary - they just hold the wood together, and don't support any weight themselves. Not sure if that makes any sense at all.
For shelves and such, it's pretty easy to do a simple dado with a circular saw - just set the dept of the blade so it only goes ~1/2 through the wood or so, and set up some other boards as guides on either side of the blade, and just make several passes through, and chisel out what's left. I've only done this with 2xwhatevers, but I imagine you could do it with a 1x4, it would just take more care.