Dieing fish

Hi.
I am new to fish keeping Only about three months now.
I have a 180 litre / about 30gallon tank of freshwater tropicals.
The temperature is being maintained in the range 24.0 to 24.8 degrees C.
I set up the tank and let it run for 7 days.
The water was tested at a pet shop and found to be ok to start stocking.
I placed 2 mollies 1 platy 1 marbled angel and 1 Silver shark in the tank.
They were there for about a month when I introduced 5 crimson sucking
catfish, about 3/4 inch long each.
I did this because I started to get some brown algae.
They cleaned the tank up in a few days.
About a week later I introduced 7 neon tetras, 2 more Platys 1 molly and 1
more angel.
I have about a dozen plants like water wisteria, anubias, some reed like
plants etc.
I have lighting which is on a timer for 14 hours per day.
Their are three fluro tubes in the tank of two different colour
temperatures.

I have been changing 25% of the water every two weeks.
The water is dechlorinated before putting it in the tank.

1. First death occurred around 2 weeks ago, I noticed one of the mollies
gills were moving rapidly.
I noticed one of the other mollies seem to harass it often particularly when
being fed. I separated the aggressive molly into a floating breeding tank
to see if the other mollie calmed down, after three days it had no sign of
calming down so I let the aggressive molly back in.
Finally the molly died one morning.

2. A neon tetra died about 2 days after it was introduced to the tank.

3. A second molly started to behave strangely. It became lethargic. Spent
most of the time resting on the bottom of tank.
Then it stared to swim strangely i.e bump into things, twist around, drop to
bottom rapidly.
I could not see any spots or lesions on its body. It died a few days ago.

4. One of the crimson sucker fish was found dead on the bottom yesterday.
The suckers don't seem to be as active as before.
The brown algae is starting to return. It looks like the suckers have little
red dots near their gills and some on the top of their mouths.

I have test kits for PH, Ammonia and nitrites.
Ph is about 7.2
Ammonia is 0 ppm
Nitrites are 0 ppm.
Since I am new at this I confirmed these values at the local pet shop who
tested the water also.

I had a high Nitrite reading before the introduction of the tetras and extra
platys etc.
I stopped feeding for three days and added a purigen filter to the filter
tray the nitrites where back to 0 ppm within 2 days after being at least
5ppm before hand.
This filter is power head which sucks water into a tray the length of the
tank at the back and drops the water back into the tank at the opposite end.
It has mechanical filtration in the form of filter sponge, it has biological
filtration in the form of some noodle like material and the purigen bag, and
has some charcoal bags.

Some pet store staff say the fish may be stressed but cannot say from what.
They test the water and say its fine.

The only thing they seem to say may be it is the water changes.
I told them I did not know how much temperature change was acceptable when
changing the water.
I know last time it dropped from 24.7 to 22.2 during the water change.
Now I have bought two 20 litre drums and intend to fill them the night
before the change and keep them in the house and measure the temperature
before putting it in the tank.

I thought fish keeping would ease stress and I admit that they look
beautiful and you can drift off looking at them but I don't want them all to
die.

What can I do.
Please Help.