MULTIPLE FISH DEATHS

Question
Hi  Set up my 95litre tank 4 months ago and everything seemed to be going fine.  Started with 6 harlequins, waited 2 weeks added 6 cardinal tetras, 2weeks - then 6 guppies, 2weeks - 6 guppies and 1 siamese fighter - within 2 days of siamese fighter had 3 torn tails - took fighter back and got 6 more guppies. (Of note the middle set of guppies had been under treatment in the shop and weren't for sale one day, (described as (a bit flaky) by assistant, but we purchased them the following day and he said he thought they were fine.  In the last 3 weeks the guppies were dying day by day - no visible signs of any illness - water parameters have been checked regularly - ph7, ammonia 0, nitrite 0, nitrate 0- now only have 3 guppies left (one of which is dying as I write this - 3 cardinal tetras have died in the last 3 days and 1 harlequin.  Water test today still as above.  In the past three weeks - as per aquatics shop started treatment with "General Tonic" - for parasites, finrot, fungus wounds and bacterial infections - they then suggested Myxazin  - for fin,body ulcers, pop eye and bacterial infections which I used for 5 days and they carried on dying then yesterday used "Kusuri" - aquatic fish wormer, fluke and parasite killer.  Since then as above lost 2 tetras and the harlequin.  I don't know what to do - I do a 18 litre change of water every Sunday using a gravel vacuum cleaner, Temp 78degrees, removed the carbon from the filter prior to these medications 2 weeks ago - now i feel like the aquatics shop is just guessing and selling one treatment after another.  As I say no visible signs on any of the fish - but remaining harlequins and tetras look quiet and fin clamped - don't thing the tetras ate this morning.  Hate watching them die one by one - they just go lower to the bottom then lie on the bottom with rapid fin and mouth for a few hours then die.  Hope you can help even if you just say let it run its course and stop treating them.

Answer
Hi Carolyn:  What I would do is this:  I would change about 30 percent of the water today.  I would stop medication treatment because medication can kill fish and as you described the treatment plan is just guess work.  I would add aquarium salt to the water because it helps fish to breath a little better.  From what you have described it sounds like your fish tank is a bit on the toxic side or that the fish you bought were not well enough to make the transition from the store to your tank.  Treat them with some brineshrimp and tubiflex worms.  The plan now is to rid the tank of toxins while building up your fish's immune system.  The extra food/treats will also improve their disposition... I know it sounds kind of strange... but like us ... happy patients recover faster... dave