skinny mollies and sudden dearhs in all my lve bearers

Question
QUESTION: my female mollies get and stay skinny or get skinny and die .also having trouble keeping swordtails and platies alive. i have 1 pair of dalmatian mollies , and 1 pair of red wag platies that have been going strong for three  to six months but any new additions don't make it longer than a week maybe 2. i have a 90 gallon community aquarium , parameters are ammonia 0, nitrite 0, nitrate 40, pH 6.8 -7.0 temp 78. i do partials every 2 wks 25-30 gallons . i don't use aquarium salt because some of my fish are sensitive to it.filtration : emperor 400 , emperor 280 i change cartridges and carbon once a month. i feed omega 1 flakes, spirulina flakes ,frozen brine shrimp,tubifex worms,fresh greens , and occasional snails for my clown loaches. so not sure what is causing this to happen

ANSWER: sounds like stress try doing a 90% water change and leave the aquarium to sit for a week with filters on.
when you buy your new fish leave them in the bag inside the aquarium so the water temp is the same.
if you do this already put the new fish in a bucket with all the water from the bags. next every 10min fill the bucket with 1gal of water until its 90% full leave them for about  half an hour.
then if they DON'T die not a single one move them into your new tank.
if they DO die in the bucket theres something in your water the new fish aren't use to. if they die during the period your filling the bucket they might be fish from a bad aquarium or (bad batches) try switching fish stores if this is the case.
if your fish survive in the bucket but not in the tank look at the other fish in the tank for harassment and gill damage.
also look for white specks on the fish this is itch if they have them.

other problems may be the male to female ratio is off
1 male to 3fem is best.
or too many fish added in at once try adding only 4 or 5 every now and again rather then 10 in one shot.
its a giving that buying fish from a store will amount to 5 to 20%death in fish but nothing more.
look at the tank your buying the fish from.
is there other dead fish? are the fish healthy and active or skinny and slow moving? is the tank clean? or filthy and full of cloud.

try picking the fish hardest to catch for the person who is helping you its fun watching them catch em and will insure higher survival rate.
hope This helps with adding new fish to your aquarium!

---------- FOLLOW-UP ----------

QUESTION: what about the fish that are established in the tank wont that big of a water change cause new tank syndrome? do you mean just water and don't hydro the gravel?

Answer
yea don't hydro the gravel that will be fine.
when doing a 90% water change try and get 3 buckets big enough to hold 90% of the water or 65% if its your first time doing such a big water change.
try and get the water as close as possible to the water in your tank which is prolly around what 26 27 degrees? then put in conditioner and if you have some easy balance mix which will help for the good bacteria you took out. every 15 to 20 min put one bucket in until you fill the tank back up if you do this correctly and copy the water condition in your tank your fish should have little to no stress at all.
I do this to ALL my tanks and I'm 95 gallons deep in tanks and I've never had a death after a 90%water change.
then let your tank sit for a week and try to add new fish and your problem should be solved. If so happens the new fish continue to die try and find similar symptoms to try and find the cause of death. my money is the  fish aren't healthy right from the pet store and your not doing anything wrong.
by any chance are you getting these fish from wal-mart?