My friend just bought a new 26 gallon fish tank. He is using it for fresh water fish. He used tap water. He let the water sit for 1 week, with the filter and stuff hooked up and running. The guy who sold it to him also gave him a packet of some stuff which is supposed to help with new tanks. He used that also. He checked the pH and it was good. He has at least 1 live plant and one castle.
Last Saturday, he bought 10 neons - just as a test. He has a knack for killing fish and he wanted to start off easy.
Today, he has 3 confirmed still alive and once which disappeared in the castle. He will be out of town for two days and his dad will be feeding the remaining fish. He has a feeling that when he gets home, he’ll be fishless.
So, what’s the deal? He had this same problem last time I bought him a 10 gallon tank. Is there some special brotherhood of fish tank owners which curses the tanks of non-initiates?
Any tips you have for owning a viable fresh water tank would be much appreciated. Also, what kinds of fish should every tank owner have? Any other animals besides fish? Frogs? Snails? Snakes? What kinds of plants are good? Rocks? Buildings?
Would you buy the scuba diver?
I almost put this in GQ but then I assumed that fish tanks, like most hobbies, deal more with opinion than fact. If I am proven to be wrong, Mod feel free to move this. I’d appreciate if you refrain from beating me with a wet fist though.
Wet FiSH, FISH you dork.
Figures the one time ever that I get timed out, it happens while I’m trying to edit.
Tho there have been several informative threads here over the years, there are tons of sites/message boards dedicated to fishkeeping. I tend to favor plantedtank.net, tho as the name implies, it aims towards - ahem - planted tanks.
In short, 10 fish is too many to cycle a 26g tank, and neons are a bad species to choose. Too fragile/sensitive. The more plants you add, the quicker it will get healthy and stable.
Yeah, the mistake here was that he put too many fish in at once, and Neons are a particularly bad choice for a first fish.
It is essential for an aquarium to have a “cycle” of bacteria established that can de-toxify the waste that the fish produce. Think about what would happen to you if you were trapped in a phone booth with someone who had eaten beans for lunch. 
That’s analogous to what happens to fish who are in a tank that isn’t “cycled” with bacteria: The waste builds up in their enclosed space and eventually kills them.
Here is a basic intro to the idea of cycling:
http://faq.thekrib.com/begin-cycling.html
The ONLY chance the remaining fish have of surviving is if you tell your friend to tell his dad NOT to feed them and to do some water changes to dilute the toxins that have built up.
The reason you don’t want to feed them any more is that the food and the waste that it produces will only make the situation worse in an uncyled tank. Plus, fish can easily go two days without food.
As for water changes, you generally don’t want to take down the whole tank and clean it out (contrary to what many beginners think they should do). Trying to “clean” the tank that way will kill off whatever cycle bacteria is growing and be counter-productive. Instead, what you want to do is take out a percentage of the water (with a bucket or a siphon) and replace it with fresh de-chlorinated water. Does your friend have a bottle of aquarium de-chlorinator? All the pet stores sell it, and then you can use the water immediately.
Even after the bacteria is established, doing water changes on a regular basis is still a good idea to help the process of waste removal along.
Adding more live plants might help the fish too, since live plants have some of the bacteria on them and can feed off of the waste themselves. Although if any of the plants look like they’re dying, remove them since then the decaying leaves will just add more toxins to the water.
Anyway, if his neons do all end up dying (which I’m sorry to say is likely), then he can always try again with one or two fish of a hardier species. Here is a really good article with some advice about choosing your first fish:
http://fins.actwin.com/mirror/fish-popular.html
My personal favorite “almost fool proof” species is the Serpae Tetra. I have had six of them for about a year without any deaths. They seem very hardy and healthy. The only bad thing about them is that, like tiger barbs, they are very “nippy” and therefore you have to avoid putting them in with long-finned or slow-moving fish, which limits you in the other fish you can put in with them. However, they are colorful and hardy. Keeping fish is less intuitive than keeping a mammal like a dog or cat, but I think it’s worth it once you get past the beginner problems. Good luck.