Fish keep dieing

Question
QUESTION: I have an established freshwater aquarium (2 years).  I do a 10-20  water change weekly, keep the temperature at 78-79 degrees, add aquarium salt and a stress guard with the weekly water changes. I test for ammonia, nitrites and nitrates, and all are within the "safe range". I use our culinary water which comes from a spring.  The Ph is quite alkaline.  About 9 months ago my husband just dumped some new fish into the tank, water and all (they were floating in the bags to acclimate to the temperature), and I very shortly thereafter had a fish plague that killed 75 of my fish, and now, I can't keep anything new alive for more than a week. I don't see anything apparent in the survivor fish. Any suggestions?

ANSWER: Hello Tamara,
First of all, thank you for including some detail. However it would also help greatly to know the size of your tank, how many fish you have and what kinds. You say you had 75 fish? That is quite a lot, so I'm assuming you have a very large tank. If not, then there is your problem - you put too many fish into an already overcrowded tank, threw off your biological filter, and killed your fish because of stress and/or ammonia poisoning. Then your tank would have to start re-cycling all over again.
If your tank is rather large, and you are 100% positive it is not overstocked and all the fish are fairly small, then I have to assume it was the "new" fish that did it.
Do you happen to have a quarantine tank at home? It's a good idea, especially because if you do happen to get a sick fish, it will develop outside of your tank and your other fish will be safe.
I recommend continuing the weekly water changes, but until you see improvement, step it up a notch and do larger ones (around 40%).
You might try a cure-all medication that is safe for fish that may not be sick, such as LifeGaurd. Always continue medication for about 3 days after your fish seem healthy just to be safe.
On that note, it is hard to tell when fish are sick until they are in the last stages, because they hide their illness for as long as possible. It's a survival thing. So you will have to go based of your fish's behavior, coloration, how their fins are looking, anything out of the ordinary, and of course water testing. I realize you said you did test your water, which is very good, but I do not know how often you are testing your water, or when the last time was. It can change pretty quickly. So, at least until your fish have recovered, it would be a good idea to test at least every other day. There are many websites on fish illnesses that also describe how to diagnose. If you feel you need a specific medication instead of a cure-all, I recommend checking those out. Just type Fish disease into a search engine and you'll be on your way.

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QUESTION: In clarification,  for some reason my percent key is not uploading, so I lost 75 percent of my fish.  I have a 39 gallon tank, had about 20-22 fish in it, and went down to 8. *sob*  All of my fish are and were quite small, except for two medium sized (3" now) algae eaters.  Neons, danios, platies, etc. Generally pretty hardy fish.  I do not have a quarantine, had never needed one before.  How long should they be quarantined?  I test once a week, and it has always come back within normal parameters. When I add new fish now, the only change that I have been able to observe is that near the end they stay near the top and in a corner.  Fins look normal, they eat pretty normal until close to the end, and I can see nothing on them (spots, fuzzies, etc.) either before  or after they die; they are not gasping.   I will step up the water changes to 40 percent.  I will also step up the testing, too.  Do you think that my spring water is OK?  Somewhere I read that well water was not a good idea .... Thanks.


Answer
It depends on which kind you are using. As long as there is no chlorine in the water you are using, and it has all the basic minerals it should be fine. You may want to use a conditioner such as Amquel or NovAqua just in case, though.
Sorry for the misunderstanding. Do you normally test before or after water changes? There can be quite a huge difference in what the results will be based on when you do the tests, so that may be a factor in why this seems like a difficult nut to crack.
Well water can be okay, but it's usually better not to risk it. I have had many problems with well water in the past, and many complaints coming to me about fish kept in well water. Usually it has to do with high alkalinity, because the water is sitting underground for a length of time. For lake fish this can sometimes be great, but for river and stream fish it's unhealthy.
See how the extra water changes help and keep an eye on the water quality before you change it out to see if that may be the problem. Let me know what happens.