Urgent advice needed please.

Question
QUESTION: Please help me diagnose what killed guppies, so I know if it threatens anyone else. My 60 litre tank was fine until I stupidly added them 2 weeks ago, with 3 zebra danios, 2 leopard danios and a pair of ancistrus. Guppies seemed really healthy at first, but have all now died horrible deaths in quick succession. Symptoms started with occasional flicking. Faeces white/transparent, trailing and hairlike. Fish sat still, either on bottom or near surface, often on top of pump. Fins & tail split, ragged. Top of mouth eroded. Some had very slight sores. Little movement followed by death c.3 days after onset. Treated with Myxazin, tonic salt & aquatic tea tree. Nothing fuzzy, filmy or spotty evident. Need to know what I can do now for the danios and my beloved bristlenoses. (horse.stable door. bolted. We learn the hard way that quarantine is actually worth the effort.) They seem okay, but I noticed that one of the zebras is trailing transparent strands of faecal matter, that is algae-like, with tiny brown specks in it.  I am starting to really worry now. Should I treat for Internal bacteria? Please Help !
Ammonia 0 Nitrite 0 Nitrate 12 Water very hard and alkaline. Temperature 24.

ANSWER: Nancy:  Try them on tetracycline and treat the whole tank.  If you can not get tetracycline try another antibiotic.  I will do some research on this and get back to you.  I think you may have a combination of a couple of things going on... dave

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QUESTION: Thank you Dave...anything I can find out is going to be helpful. Zebra still a bit iffy, but I think I might have found a fish~man with a microscope who is going to have a look this afternoon. I'll let you know, but I think my instinct is that it's bacterial. The guppies probably brought it in I know, or it was already there and they were just more feeble and prone to it. Unfortunately all the meds have probably stressed the others. Going to do another water change to try and get rid of the residue, so that it's ready to add antibiotics if necessary. Bit and Bot the Bristlenoses hate having stuff in their water. It makes them really miserable.
Thanks for your interest and your help. I'm new to this tropical fish thing...Goldfish don't go wrong that often. I see real potential for obsessive paranoia and high expenditure here. Hey ho. Anyway...I'll let you know, please  do likewise if you find out anything that might help!  Nancy.

Answer
Hi Nancy:  Most of the fish diseases that we deal with such as ICH or fungal/bacterial disease are already present in the drinking water.  Stress is the biggest cause of an outbreak. Buying new fish is very stressful for the fish and many times they end up sick as soon as you get them in your tank.  This is not because they were sick when you bought them but mostly because of the ordeal they have to go through to get from their tank to yours.  I think sometimes people do not realize that in the fish world its eat or be eaten and going from an environment where the fish realizes that they are safe to a tank they do not know will almost always stress them out.  In addition just catching a fish, bagging it, and then the ride home would be enough to kill most humans... so more then likely the guppies were fine when you bought them.  I have seen fish get stressed out even during water changes... but the more time you spend with the fish the more they trust you.  Guppies in the past for me have been terrified of everything especially the gravel vacuum but after the six or seventh water change... they practically help.  dave