What's Wrong With My Blue Platys???

Right this is my first post. I don't really know what I'm doing but have had to resort to this as a last bid to save a fish!
We've had our fish tank now for about 7 weeks. We cycled it with fish who are all living happily. Although we thought we purchases done male and female platy, the other has grown to be a male so once the tank was cycled we had are water checked and added a plec, 3 guppies and a blue platy over the week. The plec and guppies are doing brilliantly. The blue platy never really spread her top and bottom fins and seems to hang around the surface a lot. That was Friday the day we put her in. By Saturday she was hiding at the bottom under bog wood and in artificial decor. Sunday morning she tried to come for food but was swimming all over the place really off balance and couldn't get straight. She died very quickly once that started. We took some of our tank water into our fish shop and they assured us our water was fine and maybe it was shock? We bought another blue platy to replace her and a Mickey Mouse platy. Acclimatised them as usual, 20 mins floating in bag then small cups of water for a further 20 mins. Mickey Mouse doing great but again blue platy is going down hill the same day she wouldn't spread her fins so I did a 30% water change. Then the next day two 20% water changes one in the morning one in the evening but still no luck. She isn't bottom sitting but sits at the top above the filter. She's not gasping at all. But after daily water changes since Sunday evening she's not picking up. Nitrites are at 0, nitrate 20 and 0 ammonia. All other fish are happy.
There's an air outlet, a heater at 26 Celsius and an Eheim filter that came with the 55l tank. These are the only fish deaths we've had. Any suggestions would be greatly appreciated. Thanks in advance!

Your water stats look fine.
It's two blue platies that are the problem, and from the same shop? It could be a sickly batch of fish. The fact that your other fish are OK does suggest this.
Is your water soft or hard? Your water company's website should give that information, or simply, does your kettle or shower head get furred up - hard water causes furring. Platies prefer hard water, so a fish that is a bit iffy might not cope well if you have soft water.


I need to ask, the first two platies were both males; the third (a blue) was female; then you got a replacement blue which was also female and a mickey mouse - what sex if the mickey mouse? With livebearers, which include platies, the males chase females constantly and can harass them to death. This is why they should be kept with at least 2 females per male, to give each female time out while the males chase another female. This could have contributed to the first female's death, especially if she wasn't too good to start with.


Silly question - is the tank an Eheim Aquastar? I have one, it's says it holds 54 litres but it doesn't. That 54 litres is filled to the brim, I can only get 45 to 50 litres water in mine. If this is the same tank you have, you need to take that in to account when working out your stocking. For instance, there are actually no plecs that are suitable for my tank, not even bristlenoses 

Oh also, no the tank is not an aquastar, but I know the one your talking about! A friend has that one, il let her know