dying tropical fish

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: Hi Luke,
Are you sure your water parameters are safe? Sometimes people will say that low levels of ammonia or nitrite are "OK" or in the 'safe zone' when in reality they most certainly should be ZERO. And nitrates should be less than 20ppm.

Definitely double-check that factor and if there is a problem. 50% water changes daily will help save your fish.

Its possible another fish could be killing your others also. Some possible candidates are Tiger barbs, Albino sharks, and possibly the blind cave tetras. Probably the best way to tell is if a recently dead fish looked to have its tailed chewed or otherwise show injuries. You must of course catch this before the fish start naturally scavenging on the poor little dead critter.

If the fish were in perfectly shape and form then I'd likely blame either poor water quality, old age, or internal problems.

It may take a little detective work to figure this one out.

I hope this helps!
Karen~

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

QUESTION: HI,
Many thanks..
Is it possible that I am underfeeding them? They do eat everything I give them? How about the possiility that the tank is overstocked? I had about 30-40 small fish in 180 litre tank. 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
Hi!

Its really almost impossible to underfeed fish. If they are cleaning up everything that you feed them then they are probably alright in that area. You are on to something about the tank being so well-stocked with that many fish. Overstocking would of course lead to overpopulation which is the main reason why overstocking is so dangerous.

The bubbler should have nothing to do with the fish deaths. I don't think it'd disturb their sleep if they still have a quiet area in the tank to get to.

I hope you can solve your fish loss problem soon.
Karen~