dying fish

[lum]

hi and please help me!
last night i found three of my fish had died (2 pictus catfish&1malawi blue or something) although they seemed perfectly fine in the morning. tonight when i came home my two silver sharks seemed listless and were being picked on and nibbled by the other fish.i`ve had a good look at all the fish and there seems to be no indication of any disease but i gave the tank a dose of white spot/fungal cure last night just to be safe.i`ve transfered the two sharks to a spare tank but they are just sitting at the bottom with out much movement.
here`s a run down of my setup:
32 gallon tank
fluval 2 filter
1 golden gourami
1 silver gourami
2 diamond sharks
2 silver sharks
1 angel fish
1 large plec
2 parrot fish
2 elephant fish
3 tetra barbs?
1 malawi blue?
1 snake skin gourami


ph 6.5
ammonia 0
nitrate 0


could some please tell me what`s killing my fish and possible give me a cure.

ps iv`e just discovered my spare filter isn`t working.will not having it affect the two sharks? oh and i`m fairley new to fish keeping (about 9 months).

 

[Gibbo]

Seems to be overstocked and underfiltered, has the filter got a venturi (air/water mixing device) fitted? Could be lack of dissolved Oxygen....? What about ornaments/substrate?

 

[lum]

thanks for replying. substrate is natural gravel with plastic plants and acouple of lava stone sructures. i clean the gravel while i`m doing a water change (15%) once a week and the filter does have a ventura (i think i`m using it properly).as for over stocking i`m only following the advice of my supplier. maybe i worked out the capacity wrongly i have a 2.5`by 1.5`tank. if it is wrong,how many fish should i have? by the way the two sharks have both died but all the other fish are doing swimmingly!