Danio's need alot of room. Your tank is a little small for 6, especially if it is a 20H. Danios need room to swim back and forth. If they are too crowded they will get nippy. Also, it sounds like you added way too many fish at once for your sized tank. Test for an ammonia or nitrite spike in the next few days to 2 weeks. You should be OK if the tank is established, however its still risky to add alot of fish at once. For 20G cycled no more then 4 danio sized fish should be less then 2 weeks at a time, this is just safer. How many cory's do you have? What is a moon? How many moons do you have? What kind of Pleco? Your tank sounds overcrowded. A valuable suggestion also is to use a QT tank or minimum 1 month before adding the new fish to your aquarium. I have been "lucky" with adding new fish straight to the aquarium over the past 25 years. I finally learned my lesson only a few years ago after bringing disease in and really messing up my aquarium for a while.

woeisme-
first of all, i bought the tank about five weeks ago and put in the moons at the beginning. i just put in the others, so, no, i did not rush into it. i waited for the cycle to end. i have four corys, (one per five gallon) and they don't count with the fish inch per gallon thing because they clean their own waste. there are several types of moons, like tiger moon, neon green moon, etc. i don't really know how else to explain them. i don't know what kind of pleco, it's black and gray and spotty. i'm a little tiny bit overcrowded, but so far they are doing great.
- tighebettalover

Tighe-
I was confused by this- "I have a 20 gallon with moons, a pineapple swordtail, a pleco, some cory catfish, and zebra danios. Everything but the moons were bought on Saturday and Sunday"

It sounds like you added the pleco, swordtail, cory's and danios over a 2 day period. Safe stocking when going by the inch per gallon rule is 2-4 inches of fish per 10US gallons of water. Cory's most certainly count towards bio-load. Cory's will eat left over food scraps but they dont eat other fishes poop as well as their own. They still need room (at the bottom) to swim and use oxygen. Whoever advised you Cory's don't count towards bio-load missinformed you. They are just scavengers not filters. A common pleco grows up to 18"+. Dwarf plecos can be as small as 4" at adult stage. With the 4 corys and pleco (even dwarf sized) your bottom is overcrowded for a 20 gallon , even a 20L (L=long, H=high). If using the inch per gallon rule your fish stock looks like this.
1) 4 Cory's = 8"
2) 1 dwarf Pleco = 4"
3) 1 swordtail = 2"
4) 6 danios = 12"
5) moons @ 2" ea. ( I guess with your betta lover name you have female halfmoon bettas. The only other "moon" fish I can think of are saltwater , moon wrasses)

This totals 26", when they reach adulthood, of fish + the total for the "moons". That is overcrowded. The main concern though is if you put the danios, swordtail, pleco and cory's in over the 2 or 3 day period. Bacteria can multiply quickly and catch up sometimes but is safer to stock slowly. Sometimes the many bacteria's that keep your aquarium cycling can get unbalanced with such a drastic addition of ammonia. What will happen is the faster growing bacterias will colonize and multiply quicker then some others. This causes the inbalance and possibly a system crash.
I am just giving you a heads up on this, to look out for an ammonia spike or nitrite spike. Indications that something is wrong also is milky like cloudy water, a drop in pH ammonia like smell instead of an "earthy smell".
Nothing is 100%, this could happen and frequently does. This is why patience and slow stocking is reccomended. I would I.D. your Pleco to make sure it isn't a larger type one. They grow quickly and can grow 6"+ the first year.
Good Luck.

okay, i don't know why moons aren't coming up. they are small, about two inches, and i never heard of them before. must be a man-made breed or something. anyway, they're cool.
i do love bettas, but i only have stormy and tighe, those two male bettas.
i think i have the larger pleco, i'll see if i can trade it in for the smaller version. the small one will clean up just as good, right?
i'll monitor the water, and if it begins to become unsettled, i'll take back two of the danios. how often should i test the water?

I would try to trade the pleco in for store credit regardless of which it is. It's much easier to maintain the health of a slightly understocked tank. Four corries is plenty to eat any extra food, and if you're really worried about algae, just set some pond snails loose in your tank. They *love* algae, and they're lighter on your bio-load. They're also a great indicator of how well you're maintaining your tank.

My search for "moons" also came up with nothing. I think your LFS must have made up the name, or else their supplier did. What do they look like? Did they give you any information on them? I'm really curious about them now.

uninformed kitty
okay, so i already have a ton of snails. now i only have two cories, the others died. i'm going to keep the pleco, and if he gets too big, i'll sell him or get store credit or something.
the moons, as i just learned, are just platys. moon was another uncommon name for them. hokay, thanx for ur help!
tighebettalover

To your orignal question Zebra Danios not schooling:
I bought 12 zebras for my tank and they also just zip around and eat all the food but do not school. I asked this same question on another forum and what it came down to was that without predators in the tank to keep the zebras bunched up they seem to swim free without a care.

edhall is correct, I had a 240 at my disposal with 10 ZD's in it, they did not school at all, then I added some larger fish that begant to eat them, and they learned to school rather quickly and were then eaten.

Inch per gallon rule is not really all that great, as you get more and more into fish keeping you realise the folly of the rule more and more, Corries are not a large addidion to the tank, but they are an adition, they do eat alot of excess food, which is good, but what is better is if there owner does not over feed the fish, and Ghost shrimp are an even better option that corries if feeding proper amounts is not an option. If you want a fish that schools rasborahs are a much better option that tetras or danios. With the cycle and adding all those fish at once you must understand that just because the tank had enought bacteria to handle the platies does not mean it has enough to handle all the fish it can eventually handle. Also do you have Algea? Plecos eat algea and if you don't have it they starve to death.

Yes, we have a bit of algea, but the pleco doesn't seem to be eating it much. It's the green kind. He likes eating off of the fake plants too, is that weird? I have this plant that has leaves in a line, and they bunch up in one place, and he hides inside the leaves. It's a very good hiding place, but I don't see what he has to fear, all the other fish are smaller.
Oh, my second pineapple swordtail died. The first one died a while ago, so I got another, and he died. The assistant told me they aren't that hardy, so I got a very pretty molly. He's doing fine, and fits right in with the other fish. Sorry, in the future, I'm going to add the fish more slowly when I do.
Another bit of news, good this time. One of the platys had babies!!! I saw two, and caught one and put it in the breeders net. I couldn't get the other one, he was hiding underneath a rock. I already have another baby from another platy, and she is getting really big. But I'm probably going to only keep one of them, and give the other to the petstore. I'm hoping the new one is a guy, because I only have one male platy. It's funny because although my male is the smallest, he bosses everyone around because he's the male. It's quite funny,

I've seen about 20 different green kinds in the past 3 years, You'll have to be more specific; Its fairly common to eat off of the leaves of plants, fake or real; He hides because its in his nature to be reclusive. I wouldn't bother to buy any more fish for atleast a month, when the bacteria should be caught up. The male Platy isn't bossing the others around, hes tring to mate with them; Swords and platies can breed with each other; Giving baby fish that are not rare to the lfs is not a reliable way to deal with them, you and every other new aquarist with platies out there has babies that they feel like need to be saved.

Watch out for the bossy danio fish. I have one that I had to put in its own container because it tried to kill the other danio and severely hurt a golden killifish that was very gentle. It started with just being bossy and chasing the other fish, leading up to attacking the other fish from underneath and ramming their stomach area like a shark.

Your tank was not too small for the zebra danios. I used to have them before and they never schooled together. I highly doubt that they ever schools together, because they like to swim around like crazy everywhere.

i understand that this post is 5 years late and im posting it for the sake of those who come across this thread while searching the internet.

1. a 20gal is ideal for about 10 zebra danio
2. this tank is stocked under with the exception of the pleco
3.danios of any kind DO NOT school, they shoal
4. a 20gal tank can hold a maximum of 4 cory
5 If you ever have platy/sword cross breeds and return the fry to the store, YOU MUST tell the store offical that they are cross breeds because no one wants them most of the time being that their usless in the trade, sorry but mutt fish are frowned upon.

Never have I ever heard of platys being called moons. There is the mono fish (looks like a mini angefish crossed with a barb)

The mono fish I've heard referee to as a silver moon or a moonyfish or moony. But typically they shouldnt libe in freshwater tanks. They do way better in strong brackish water or even better would be a marine aquarium.