Hi all hoping someone can help.

We started a 20 gallon (24x16ish) tank about a month ago with 5 zebra danios, all went well, it cycled ect. We got back from a vacation and decided to start adding more fish. We had thought to add some Glofish (cause they look cool! and they are also Danios so they should help us get up to a schooling size). I got 2 red ones yesterday, we added them in and then this morning I noticed LOTS of aggressive behavior.

Before I would typically see 4 of the 5 swimming around happily with one hiding except for feeding time. This morning it seems like 2 of the zebras are being very aggressive with each other and all of the other fish. No one seems to be able to hid and be left alone and everyone it just swimming around either instigating or running away.

I have not tested the water since we returned (I will do that this afternoon). I did notice the two aggressive zebras seem to have reddish around the gills, so we will also probably to a water change for good measure.

The tank is fairly well planted, not super heavy but 6-9 plants, and a few places to hid lower down. I didn't think adding the Glofish would mess up the schooling since they are the same species. I was also thinking about adding green/orange this weekend when the store gets more in... I know to be careful about overloading the tank, but I thought it would be ok, maybe they are feeling stressed in the space :( Maybe they just need to settle in.... :(

Any help is much appreciated!