I added 3 zebra danios to my 40 gallon aquarium about a week ago. They are the first fish I added to this tank, and my girlfriend brought over about 12 swordtail fry since then. Anyway, the first day or two that I had the danios, they were active as the should be and exploring their new tank. For the past 5 or so days they've been hiding in the back and really only come out if it's dark or if they can't see me.

I don't want them to be stuck hiding in the corner of the tank their whole lives so I was wondering if there was anything I could do to make them more comfortable coming out. I can't even really entice them with food, they don't see terribly interested most of the time.

I could be wrong but they are a schooling fish. You really need a minimum of 6. I would say that is why they are hiding. Are your water params good?

Another thing but I'm sure you good with a 40gal, they need room to swim so make sure there is plenty of free area.

i don't feel my tank is established enough to get any more fish. I didn't even want to add the fry but my girlfriend brought them and she lives an hour and a half away so I couldn't realistically ask her to take them back up with her.

I'll consider it for the future though, once the tank gets more established.

I can't comment on the swordtail but the danios will be better off with 3 more, they are stressed at the moment and you don't want that. They are a hardy fish and done right your cycle will benefit from it. You are obviously still cycling? Keep 6 of them and feed small amounts 3 times a week. Keep ammo bellow .5, ideally kept at .25. If you want further info please google "good fish to cycle a tank" or something like that. 3 danios will not create enough ammonia anyway. Well at least I don't think.

Two possible factors here,

1) As others have said you need at least 6. Zebra Danios in particular tend to get very aggressive and stressed when kept in smaller numbers.

2) Is your tank cycling? If you did not put your tank through a 4-6 week nitrogen cycle before adding fish then it is. During the cycling period ammonia spikes etc will cause your fish to be unwell and act like you are describing.

The way to solve the first problem is to get more fish but If your tank is cycling that might not be a good idea. Do you have a test kit? In a cycling tank with fish in it you should probably doing 50% water changes at least every 3 days and be careful not to overfeed.