Immune fish and toxic water?..!

Question
Hello!

I'm a 17-year old HS graduate, and I work as a full-time veterinary assistant until I attend college next fall semester. Knowing that, our veterinary hospital only specializes in cats and dogs (and the occasional ferret!) but unfortunately not fish or exotic birds. So..I have an issue that I've had with one of my 10-gallon aquariums since its beginning.

I admit to starting it off wrong, not allowing it to cycle or run for several days in advance, but eventually the fancy-tail guppies I added bred. I currently have 5 white-skirted tetra, all of which have survived since first being introduced. I have 18 fancy-tail, red cobra guppies, and one green cobra (the father). My question is..now whenever I introduce a new fish (plecostomus just recently) they die.. The tank I had was very poor quality, and when the water continuously evaporated, I wonder if it made the water very hard from the salt I'd add occasionally. Is it possible that the water is very toxic, and only the guppies born in it can survive..? I purchased a new aquarium and I've finally finished the landscaping and I'm letting it run for a good while with water conditioner before even thinking about switching the fish over. Would it be possible for the cleaner water, once its ready, to kill them if they're used to strong and unhealthy water conditions? Any advice would be appreciated!

Answer
Hey Briana,
That's cool that you got a job in a Vet's office, are you planning on going to college to be a vet? I'm thinking about some sort of veterinary or biology career when I graduate. When I turn 15 in October I am going to try applying at a few Vets, even if it is just cleaning cages and stuff like that. I'll look into veterinary assistant, but I'm probably too young still.

They very well could be adapted to the bad water, Since they were developing in their mom's belly while with the water. They should be fine going into the new tank, but I would try adding only a few in the begining to see how they do. They should be perfectly fine, and do even better. The chance of them needing the really hard water is slim, but there's always the chance.

Be safe, and try adding around two or three of them. If both of them make it past one or two days, you can choose to add a couple more or just go ahead and add the rest.

They should all be fine and it should go smoothly with out events. It was a really smart idea to ask some one before you put them all into the tank, I get so many questions about things people already did and how to fix them...it makes everything difficult.

Good luck with all of your fish!! I hope every thing goes well! If you have any more questions, feel free to ask!! Good luck with college!
best wishes, Chelsey