Yes, most people (myself included), do have a lot of fatalities at first. But it doesn’t have to stay that way, as long as you are willing to ask questions and read (which obviously you are). Just by knowing about the nitrogen cycle, you are head and shoulders above the average aquarium owner who fills their tanks up with water and then goes to the pet store the next day to get all their fish (yes, I did it too, knowing nothing about biological filtration back when I had my first fish tank).
I don’t think you have to fear for the entire tank dying.
The problem with these first deaths might be that your fish choices are not really the best for someone just starting out.
This page talks about some of the good and bad fish for beginners:
http://faq.thekrib.com/fish-popular.html
Of course, that doesn’t mean that all is lost or anything…it just means you may need to put some extra care into keeping these particular fish.
It may be hard to believe, but Angelfish are the natural predators of neons in the wild! Some people can keep them together successfully, especially if they are kept together while young, and chances are your angels are too small to eat your neons right now, but I would be very wary of keeping a large angelfish with neons (or any other neon-sized fish, for that matter). Therefore, you may want to plan to eventually move your angels to avoid the threat of having them eat your neons.
Your pleco will probably need supplemental food. There is rarely enough algae, especially in a new tank, to sustain a pleco. This page: http://www.tomgriffin.com/aquamag/pleco2.html suggests ways to make sure your pleco has what he needs.
Others have already mentioned that danios like to have larger groups. That’s true, however, you have to be careful not to overload your tank. Usually, the rule of thumb is “one inch of SLIM-BODIED fish to one gallon of water”.
The fat-bodied fish generally produce more waste per inch than the slim ones. That’s why 10 neons will be fine in a 10 gallon but a 10" Oscar really needs a 55 gallon tank.
Now, remember, it’s a rule of thumb, not an iron-clad rule. Good filtration, lots of water changes, live plants, etc. can let you get away with more fish, while not doing water changes and so forth may actually mean having 1 inch per gallon will be too much fish for the tank.
The catfish you have…probably a type of Corydoras? You may want to look at the photos on http://www.planetcatfish.com to confirm what you have.
Assuming you have “cories”, those are great fish. Very cute and fun to watch. But again, like plecos, they often starve because people expect them to be sustained by scavenging alone. Many people feed their cories special sinking foods for bottom feeders to make sure they get something to eat. It was wise of you to get more than one, though more would be even better. Cories love having the company of their own kind and will do much better with “friends” around.
It could very well be that the hatchet fish died from stress or disease rather than a water quality problem.
To be honest, I wouldn’t recommend hatchets as a beginner fish simply because of how, um, jumpy they are (please make sure your tank cover is secure for the sake of the surviving hatchet–and the danios for that matter, which also tend to jump). As http://www.notcatfish.com/ffotm/1999_05.htm puts it in talking about one species of hatchets:
If kept in small numbers, the fish can panic easily and will leap, several feet, from the tank. Similar to their close relatives of the genus Gasteropelecus, Carnegiella species have curved pectoral fins that can be moved like a bird’s wing. This enables Hatchetfish to literally fly from potential danger. I recommend fitting a tight cover to the aquarium as from time to time, a fish will jump and land on a bracing bar - this is a most unpleasant discovery to make, particularly first thing in the morning!
Neons, being so tiny, apparently tend to distintegrate quickly after death, which probably explains why you didn’t find a body. Unfortunately, though neons are very pretty and very popular, they are kind of sensitive to water quality and so do best in tanks that are already well-established. Neons can also be affected by “Neon Tetra Disease”, an inevitably fatal disease that is contagious to other neons. It’s never a good idea to buy fish from a tank with sick or dead fish in it, but with neons it’s particularly important to look at the health of the fish in the tank. By the way, most fishkeepers who have invested a lot (emotionally or financially) in their tank keep a small QUARANTINE TANK for their new fish purchases to stay in for a couple weeks to make sure diseases don’t spread to the other fish. http://www.notcatfish.com/health/quarantine.htm will give you some idea of what’s involved with setting up a Q-tank.
Aggression seems to depend on the individual betta. Some of them do get mean, but others are fine for community tanks (all bettas are potentially vulnerable to fin-nippers, of course, with their long flowing tails). If possible, you can solve the problem (without fatalities) by getting a small tank for the betta to have to himself. The tank doesn’t have to be extravagant; while many betta keepers don’t agree with keeping them in those TINY cups where the fish can barely swim around, bettas really don’t need more than 2 gallons (as long as they are the only fish in the tank).
In fact, some people claim they actually prefer a relatively small tank: http://www.aquahobby.com/gallery/gbetta.html
By the way, just to clear up the idea that first fish HAVE to die…there is a safe way to cycle tanks! It’s actually much safer (for the fish) and easier (for both you and the fish) to cycle a tank WITHOUT fish first. Check out these two pages for info on fishless cycling:
http://www.tomgriffin.com/aquamag/cycle2.html
http://www.tomgriffin.com/aquamag/cycling.html
Also, I highly recommend that you visit a forum devoted to tropical fish. Just reading the posts already there, you will learn a lot. I know I have. Two of my favorite boards (though certainly not the only good ones) :
http://forums.aquariacentral.com/
http://forums.fishindex.com/
There are some wonderful fish experts hanging out on these boards: people who have been keeping fish for decades, professional fish breeders, responsible pet stores owners (beware that you may find your neighborhood pet store employees really dont know what they’re talking about when suggesting products or fish combinations). The combined knowledge on these fish boards is truly priceless…and yet free to all of us.
Also, as tempting as it is to “impulse buy”, you will be happier in the long run if you always research the fish you’re thinking of getting before you buy. Perhaps even ask on messageboards like the ones mentioned above if the fish you’re thinking of would be compatible with what you already have.
One good reference guide to fish is http://aquariacentral.com/species/ which conveniently gives some basic info like the minimum tank size needed to house various fish species. That’s the best way to avoid unwittingly bringing home a cute baby Pacu or Red-tailed Catfish only to find out they grow into tankbusting monsters!
Another site with helpful info on fish is http://www.aquahobby.com/gallery.html , but since the advice posted there is based on people’s individual experiences what one person says about THEIR fish may not apply to every fish of that species.
