Dead Platies, whats going on here ?
 

I'm after some advice as I'm a bit stumped on this.

In my tank already I've got 3 clown loaches, 4 neon tetras, 1 rainbow shark, 6 sword tails. And the week before Xmas I bought 8 black molies and 8 sunset platies. And there all in my 50g tank.

All the fish are happy and doing well (or look to be) and then all of a sudden about 1 week after buying my 8 molies and platies, 1 of my platies died. The only unusal symptom I can describe is about an hour before it actually died it seemed to be swimming rather odd, as if it couldn't swim stringt, its head always seemed to pointing towards the surface.

Anyway th;is week I've lost 4 more platies I'm not sure if they died in the same way but I always found them either lying dead on the gravel or floating at the top. I've now only got 3 platies left which seem to be fine and happily eatting. As do all the other fish. Theres no signs of illness all the fish are bright and eat well and very active I just dont understand whats killing the platies.

I've done an ammonia test and nitrate test and both came back absolutely fine, so unless my PH is wacky I can't think of what might be doing this, and if it was the PH wouldn't the other fish be affected ?????. I come from a soft water area so I'd have thought my PH would be slightly acidic ????

I'm stumped here, have I just bought a bad batch or do platies have special requiremens that differ from other livebearers ???, any ideas ??????

Sorry about your fish :(
If all the other fish are fine, with no signs of disease, and the ammonia and nitrite are 0, nitrates are low (<20) and the pH is steady, it's possible you got fish that were sick to start with. Platies are generally considered tough fish that aren't picky about their environment, but I've often wondered if they've become more delicate or weak than in the past, due to so much inbreeding. I hope the rest survive; if they do, you'll have replacements for the ones you lost in no time ;)

I've just had a good look at the tank today and suprise suprise guess what I saw, yep ICH !!!!. Ah what a pain, no doubt I've introduced it again with the latest batch of fish, so I'm now treating the whole tank, hopfully I've caught it in time, but I'll keep you updated.

As such I'm now considering getting a UV steralizer whats the general feeling on these devices, I've got a Fluval 204 with horrible ribbed hosing, ideally I'd like to replace this with rubber hosing and then add the sterilizer. Has anyone had any success adding a sterilizer to the ribbed hose ?

If you haven't done so already, you may want to ask this in the equipment/DIY area; you'll probably get more responses.
I don't know if you've already bought your UV sterilizer, but I'll pass on a bit of info I was given by a (knowledgeable) LFS employee when I was thinking of buying one: the units that spiral water around the bulbs are apparantly much more effective than those that allow the water to run a straight path past it. More time exposed to the light really makes a difference. When I was window shopping, I found some of the 'spiraling' type units were more expensive, but he said they're worth it.

How old is this tank??

One thing alot of people do not know is that these larger live bearers, mollies, swords and sometimes platies like larger tanks. And yes you do have a larger tank but you might be over stocked with the fish you have. Sounds funny, but Swords and Mollies and get pretty big if given the room. So this may have had stressed out your fish. But this could be due to the age of the tank. I kept a sucussful Sailfin Mollie 55 gal tank with about 9 fish for a long time before I converted it to salt.

I would treat the ick and raise the temp of the tank. Remove all filtration to the tank. But you have loaches so it may be really hard to treat the fish, since clown loaches are pretty hard to treat for this illness from what I have been told.

I'd say on top of bad stock, it would be an overstocking issue.

You're not only mixing incompatible fish, you're totally overstocking your tank.

Clown loaches get to 18" in length, although it takes awhile, they do get that size. Neons come from soft acidic water, and the livebearers come from hard alkaline water. I also wouldn't put it past the rainbow shark to be picking on other fish in the tank.
Overall for a very standard 1" per gallon rule you have over 129 inches for a 50g. My guess is that is what caused the ick, such a huge stress on a bioload that couldn't come anywhere near handling that bioload sent the water quality south. It's also possible it was just bad stock, but with all the combining factors it was a lethal combination to the tank.