sick molly

Hi. I am new to aquariums, have only had one for about 2 months.
It is 180 litres about 40 gallons.
I have 3 platys,2 mollies,6 neon tetras,2 marbled angels and 5 sucking
catfish and one silver shark.
I have a few plants (water wisteria, anubias,swordplant and some reed like
plants)
I have tested water for ph , ammonia and nitrites and all are correct I am
told by pet shop.
Ph is 7.2,ammonia 0 ppm, nitrite 0 ppm.
Nitrite was up high about 2 weeks ago but I got it down in 2 days by not
feeding and adding a purigen biological filter bag to the filter.
The water is 25.0 degrees Celsius.

I have lost 1 Mollie, found dead in morning about a week ago, its gills were
moving rapidly for about 2 weeks before this.
Another Mollie is looking sick, It is not rapidly moving its gills but it is
lethargic, lies on bottom of tank most of time and moves slowly.
It also seems to bump into things , like ornaments or the bottom of the
tank.
I cannot see anything on its body like spots or wounds as described in books
and articles I have seen.
It seems to have trouble feeding, the other fish take food very rapidly and
come back for more. This one has small bits of fish flakes and sometimes
spits them back out.
The other Mollie often chases this one away from it particularly when being
fed. I have Isolated the sick looking one in one of those floating plastic
enclosures so the other Mollie cannot harass it and so it can get to the
food I give it.

What can I do to help it?

I don't want to loose another one.