my all new fishes died?

Question
hi, i have 17 gallon fresh water tank. i bought it 3-4 months before. at first time when setting up a new tank i was having 4 pair of goldfish and 1 carp and 2 angelfishes and till the date all are healthy and strong. now their size up to 3-3.4 inch long and 1-1.5 inch broad by width. day after yesterday in the evening we bought 8 new fishes[2 silver shark,2 black moor gold,2 pearl gourami,2 lupius(i am not sure about exact name)]. they are are happy in the night(means i checked it 2-3 times in the night) but i saw in the morning my fishes loosing their tails.their Fins that seem to be growing thinner or more transparent.Fins that seem to be "fraying" around the edges.Fins that seem to be breaking off in large chunks.Slimy looking areas, usually on the tips of fins, that seem to "eat away" at the fins over time and its affected to all fishes in a very short period. then i search on web i got, its seems to be like fin rot. so i changed water up to 60% and added water conditioner, anti ich(medicine which bought from aquarium shop keeper which work like antibiotic.) then added rock salt near about one bowl, and 2 capsules(which bought from aquarium shop keeper which work on any disease as he told me) and not gave them food for two days) but i found after one day my all fishes getting died one by one and till the yesterday night all 3 pair which i bought were passed away. and right now only 2 black moor gold is alive. i am not getting what is happened to them and how it happened? because i tried my best but why my newer one died so fast? and what should i do for my older one to save them?

Answer
Hello Pooja,

As stated in my profile, all questions are published.  I have changed this to public.

Well, there is a rule of thumb which goes alongside fish-keeping of freshwater fish.  That is one inch of fish per gallon.  If your fish are goldfish, the rule changes drastically, because goldfish poop so much, that they require at least five gallons each to survive.  Carp/Koi are another story.  Each requires 50 gallons.

A 17 gallon tank cannot possibly support as many fish as you have.

Your fish did not die of a fungus or a rot.  They died from Ammonia/Nitrate/Nitrite poisoning for sure.  They literally got burned on the skin and fins from the water quality being so high in ammonia.

Ammonia is caused by waste.  Fish in a tank that many, with new fish added, are swimming in sewer.

What happens when you stand around a room filled with urine?  It burns your eyes.  Imagine swimming in fish waste, which forms heavy ammonia.  The skin burns and then rots off.  It will turn to a glutinous gel and sometimes it will literally break off.  This is what you saw.  Your fish died from improper water conditions.

In a 17 gallon aquarium, you can have no more than 3 goldfish.

You can have no carp.

You can have only as much fish as your tank and filter can support.

Use the calculations above to arrive at the right ammount.  Clean the filter.  Clean the tank and gravel and start with a fresh, new tank of water.

This is my advice and take the 2 survivors out until you are done and keep them in a good, clean bucket of water.

I hope this helps.  Happy fish-keeping.

Renee