I'm so sorry to hear that. I can't imagine having that happen, watching it, and not being able to reverse what ever was the problem. I'm with the others - a combo of oxygen loss and some chemical getting into the water.
But don't give up on ponding.
Think about increasing the size of the pond. If you can't expand it, you could use cinder blocks or landscape timbers to add height around the edges, and then install a liner. This means most of your pond is below ground level, and then you add a foot or two above ground, increasing your water content by several hundred gallons, if you want. I just saw them do that on HGTV.
I have a Pondmaster pump, (a couple, really) and they work well and are dependable. Last year, I went to a pressurized canister filter, and love it. It cleans the water well, and then pumps the water up my waterfall, so it creates a lot of aeration, too.
If you can afford it, get one of the back-flushing self-cleaning ones. But beware that a lot of them out on the market don't work. I learned that the hard way, buying two different brands that don't work. PondMaster's, with balls instead of filter media, plus a gizmo that manually agitates the balls when you back flush, that one DOES work, and I wish I had spent the money for it. On the other hand, taking my Tetra filter apart once a week and hosing off the media (Laguna media, not the stuff that came with the filter) is not much work at all.
And remember - this time around, instead of buying lots of pretty fish, be selective and only buy the totally fabulous fish that you really love. That will eliminate the chance of overcrowding your new pond.
Oh, and show us some pics.