help!!!!!!

i have a tank of about 130litres (estimate) containing 9 danios(5 zebra, 4 leopard) 3 black mollies, 3 silver sharks, a male siamese fighter, 2 albino corys, a pleco, 2 blue rams, 2 boesemans rainbows, 1 tuxedo platy and now 2 red coral platys.

until yesterday i had 4, and the day before i had 6 coral red platys. sometime during the night on sunday 2 platies died, 1 under the filter, the other in a gap between rocks about a foot away. last night, 2 more died in exactly the same places! the platies which have died so far were bought about 6months ago so they aren't old, but were from a different fish shop to the two remaining platies. the fish havent really been acting strangely and have been successfully breeding. there doesnt seem to be anything wrong with them until i wake up to find them dead, the only thing i can think of is that the ulcer which killed a danio a week ago was more serious than i thought.

none of the other fish are acting strange or have any ulcers present. their diet is varied between flake and live food. the water parameters are all fine for this area- nitrate 10ppm, nitrite 0, gh >16, ammonia 0, ph 7.6.

i need to find out what is causing this, as i dont want to wake up every morning and find more dead fish. any help is much appreciated!!!
kim

so why are the platies dying?my sharks and pleco are nowhere near fully grown, none of my fish are over 3 inches.
one molly is male, other 2 females. the platies were in the ratio 4:2 female:male, now only have one of each. i dont know the exact capacity of my tank bear in mind, its a 4ft x 1ft x 1.5ft.

yes my tank is planted, has been since the start.
we use liquid tests occassionly but mainly dipstick tests, these seem to be more reliable and easier to use. the error rate with liquid test kits is reportedly higher than other testkits so i do try to avoid them.
as far as i was aware, my nitrate levels should ideally only be just above zero anyway. any higher and its a sign of poor water quality.
as my ammonia levels indicate, my filter is perfectly capable of handling the waste produced. i've been away too so my water hasnt been changed or filter cleaned for 2 weeks and my parameters are still good.

nope, the tuxedo platy has been a bit randy lately, but nothing too drastic. all the other fish are peaceful with each other!!
so its got nothing to do wit the ulcer on the danio???
if thats the case then iv got nothing to worry about then!

course its worrying, but if its not a disease its less worrying, as i dont have to go around buying medicines and hoping it works before my fish all pop their clogs, get me drift?

so how the hell you meant to know whether your overstocked or not if the water quality dont change to give you some kind of indication. my petshop always said so long as the waters fine, the amount of fish you have is fine! ive had the tank setup with this amount of fish for at least 2 months and none of the fish have previously died.

if they were being subject to aggression, there would have surely been some kind of indication before now?and there would be signs of it, the fish and their fins are completely untouched and unmarked. i've had aggression problems before with my bumbleecatfish, so i know what to kinda look out for.