No problem, but may i add something;
An undergravel filter only provides Biological Filtration.
With that extra 100 i'd buy an Aquaclear, hang-on-back filter to at least add the Mechanical Filtration. An undergravel filter itself really isn't enough, especially because it doesn't catch free-swimming dirt, but rather, only dirt that has settled into the gravel.
And i'll be your Fish store and tell you why it died: Ammonia
Like i said though, i wouldn't recommend adding anything, at all until a month has gone by. By this time, your tank will have cycled and it will be a happy environment for other fish, rather than getting all of your fish now, having them struggle with life through the cycle ( And i doubt any bottom feeders will make it, generally speaking, only the platies might), and then them having a permanently weakened immune system.
And also, by adding so many fish at once, even as cyclers, it could lead to an Ammonia spike, utterly killing everything you've bought.
The filter may cost you about 20 pounds. If you really want your tank cycled, and want to add fish, i'd look out for Bio-Spira. This is the only product on the market that actually cycles your tank in two days. Don't let your eyes stray from this because of the price, i'm almost certain nothing else actually works.
If you wanted to add plants, i wouldn't recommend it, again, because of your Undergravel Filter. Having a water current pass over the plant's roots is alien to them, which will in the end kill them. I can honestly think of nothing better to spend that money on than the filter, and Bio-Spira. Unless you were getting fake plants and other deco, (driftwood), but if i was you, i'd at least set some money aside for the two.
Hope this helps,
mike
Oh, missed this:
The water may look clean, but that's because ammonia is clear. You wouldn't be able to see it, nor Nirtrites or the Nitrates.
And in fact, now that i think about it, the bottom feeders would interfere with your cycle. Yes, they'd clean up the gravel, but that would minimize the amount of ammonia in the tank on which the bacteria has to feed. You really need that food in there for that roller-coaster-like first month