Tropical fish deaths

Question
QUESTION: Hi,
I wonder if you could help steer me in the right direction.
I have had my tank up and running for a year now, its 180 litres and contains a variety of fish. I change the water weekly and test the water regularly for nitrites/nitrates, gh/ph levels and all are within the recommended levels.
I have a filter running and temperature is a steady 26.
My tank has been reasonably happy over the past 6/7 months with very few deaths, however over the past 2-3 weeks I have lost about 15 fish, most mornings I awake to another dead fish. Ive taken the water to the local aquarium and they tested it and said it was a little hard and that I should reduce this, but it wouldnt be what was killing the fish. I did point out that I had some blind cave fish, and the shop advised that this was almost defintiely the problem and that I should remove them, which I have done. However, fish that have been added since the blind cave fish have been removed have now also died. I am at a loss as to what the problem is, and having spoken to my local areas am really at a loss as to how I can resolve this problem.
My tank is severely depleted and I am really confused.
Any advice you can give me would be really appreciated.
I currently have 5 adult Mollies, 5 babies (all less than 2 months old). 4 adult platties, (4 babies). 3 guppies, (1 baby) 2 cherry barbs, 4 golden loaches, 1 apple snail, 1 abino shark, 7 tetra embers, 3 danios, 4 tiger barbs and 2 plecs.
thanks in advance
luke

ANSWER: The cave fish wouldn't be the problem, they are just like any other fish. More likely is it the tiger barbs and plecos.

Tiger barbs are agressive and should only ever be kept in species tanks, unless they are in schools of two dozen or more. I would remove them immediatly.

Most plecos will get over 1 1/2 long and need a 350 litre tank. Not an immediate problem as they grow slowly but I would also suggest taking those back to the store.

Now, tell me what fish you had prior to these deaths? Which fish were the 15 that had died?

-Nick

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

QUESTION: Hi,
Thanks for your swift, and good response. It surprises me how different answers can be, even the guys in the shop disagreed on the blind cave fish.
ive lost platties, guppies, a dwarf gourami, a red tailed black shark (biggest fish in the tank), 4 green tiger barbs, 1 angel fish. i think that covers the different types ive lost. none of the fish look obviously ill either..

ANSWER: I think you were just overstocked before. A single pleco produces the same amount of waste as a half dozen tiger barbs. If you want to keep the pleco's it's fine, but chances are unless you have a bristlenose or another dwarf pleco, you'll need a big tank soon.

That about covers it. Get rid of the plecos, the tiger barbs, and it wouldn't hurt to increase water changes to 25% twice a week.

-Nick

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

QUESTION: HI,
Many thanks..
Is it possible that I am underfeeding them? They do eat everything I give them?  I have also addded a machine that blows bubbles from the bottom of the tank, could this be upsetting them? (they do seem to like playing in it if anything, but Im wondering if leaving it on all night disturbs their sleep perhaps?
Any ideas?

Answer
Learning how much to feed is an aquired skill. You ideally want to feed enough that you can see a little difference in the size of their tummys, but not so much that it is bulging out extremely. A little bump is ok.

The bubble maker doesn't have much use, but leaving it on won't disturb the fish.

-Nick