hmmm guppy problems

ok so my friend did hand over her tank, complete with 6 corys and 14 guppies +fry,around 20 i think, anyway,i separated males from females,and rehomed them along with my spare 3foot tank to another friend,as i coouldnt think of stocking options for it. anyway, i kept the 6 corys and 6 adult male guppies in the tank i have with the 20 ish fry which are around 1-2weeks old and growing well. yesterday i lost 1 of the male guppies and ive just noticed and removed another one,they seem well and healthy, no disease, water parameters are ph 7.5, nitrites 0 nitrates 10-15 ammonia 0 temp 79-80. the dead ones look fine, no visible problems, they just seem to sink to the bottom and stop living the one i just took out was a bit contorted and his fins were all stuck right out with his mouth as wide open as it could be,almost like he was gasping for breath :help: all the other fish seem fine and well, but then again he was well and healthy,eating and swimming with the others only a few hours ago! is there perhaps something in my water that could cause this to happen,its just i want to get a couple more fishies in the tank,but if guppies dont survive in my water a guppy only tank isnt going to work!!! the corys thrive in my water and spawn regularly, and all the guppy fry seem to be doing really well,i would have thought it would be them struck down first with them being to smallany help greatly appreciated.

ok,so tank they are now in, is my old tank, fully cycled for months now, i rehomed the females with the other tank from my friend, but i did move the tanks around, so either way, filter remained in tank (my old tank) and tank was moved from one stand to another, as i wanted it in different place, so it was almost completely emptied, had about 2 inches water in it and the sand, filter was put into clean bucket of old tank water, bucket only ever used for fish, never had any problems with other tanks, with guppies and cories in it aswell, i vacuumed the sand and stirred it up as usual, making sure no gases build up, then refilled tank with dechlor treated tap water, at about 78-80*, ran heater and filter in it for 10 mins after set-up again, added filter boost stuff, that prob doesnt do much but i kinda got in the habit and cant seem to stop lol, so tanks all set up again, all decor replaced after wash off in old tank water, everything settled then added fish back once temps were the same, but i did the water/tank change over like 3 days ago, so i cant imagine anything has happened due to that. i use a liquid drop test kit,that i cant remember the name of right now lol, its a master test kit i think,cost about £20. its fairly new,only had it about 4 months so not out of date. right now my cories are happily mooching around the tank as always, sifting the sand etc,the baby guppies are swimming around plenty and are picking at flakes when i add them twice a day,but the adult guppies are all now hanging out at the top of the tank, just sitting there,not swimming much,and looking like the others did over past 2 days before they died. i just dont get why the babies are all doing so well, colouring up nicely and growing fast when the adults just die!!! i really dont want to lose my guppies,they are beautiful fishies, they have been flashing since i got them,but its only every now and again and theres no sign of disease. have just tested water again, nitrite 0 ammonia 0 nitrate 15 ph 7.5 i just dont understand,ive never had luck with platies either-maybe it is just a livebearer thing! help.......

oh and guppies were placed into fish bags and were in there for no more than half hour when they were brought over, and were not netted they were gently scooped in a clean plastic jug and placed into the bags,and water was tank water.

well i woke this morning to them all still alive,though 1 is just kinda hanging again at water surface. when i put food in they do still go nuts for it though,im thinking i may go for some neon dwarf rainbows in the tank instead of the guppies,i will take the 5 remaining males to my lfs and swap them for some rainbows, the guppy fry will be eaten by them i would have thought too. ive never had luck with guppies or platies, but my old dwarf rainbows did really well,so i think il try them again,plus they are really beautiful fish!