What can I do to keep my fish alive and healthy?

Yes, these are all schooling fish. I would return the catfish. They can reach 4" in length, and a school of them would nearly fully stock your tank. You might get by with a school of each of the other two species, as they aren't very large fish. The Tetras can reach up to 3", and the Danios, about 2". Just so you know how large your fish will be when fully grown. I think a school of each species could live comfortably together in your tank. I would not add anything else, though. If you plan to add something else, return two species, and get a full school of just one.

It shouldn't be that difficult. Give us some specific information: How big is your tank, how long has it been running, what kind of fish and how many are in it, what equipment do you have with it, what's going on with them dying? Really, fishtanks are not that difficult to maintain. There's a 3 month period at first when you set it up called "Cycling". That's where you have to allow the good bacteria to cultivate and colonize your tank to eat the amonia and the nitrite. After that, you pretty much need to feed and do a weekly water change of 25% and a gravel vacuuming and you should be good to go. The only other stumbling block is that you may have gotten too small of a tank. The larger the tank, the easier it will be and the more fish you can stick in it. Ideally, you should stay at 20gal or above. You also have to make sure your filtration is adequate for the type and amount of fish you want to keep in there. Don't get fish that get too big for your tank (Angels don't belong in anything less than a 38, Goldfish don't belong in anything less than 20gallons and you can only stick 1 or 2 in there, Oscars need a 75gallon, etc)

you will find the danios are too active for the tank, but otherwise you're fine.

Are the "glo fish tetras" actually glofish danios? Or are they glolite tetras. If they are tetras get 5 more or so....And get 5 more glass catfish (they're very sensitive, you may have trouble keeping them alive)

What's more important is learning how to properly cycle the tank, keep excellent water quality and proper maintenance procedures. If this hasn't been done then these fish aren't going to make it anyway...

You have enough space to add 4 more of each. Glass fish only need 3-4 in a group(social) and really need 30 gallons (30 gallons tanks are not the same as standard 29's, they are much longer), but as long as its kept in a proper group can thrive in a 29 gallon. Danios And tetras need groups of 6+ as they are schooling fish. Just make sure you dont add any more fish after that, your tank would even be a little past its limit.

It's okay, what you have is just fine and don't worry about fishes going school with, they don't! Fishes in the aquarium especially fresh water fish don't group up inside an aquarium. Been there, done that so need for you to worry of taking some of the fish out of your tank. If your concern is just to keep them alive, make sure that the oxygen level is always right by making sure that all your pump is working right and filtering and cleaning must be regular. Do not over feed them and always remove left over. :)

You seem to be fine.

no