the water is oxygen poor and there is more O2 there. (How many days/weeks did you have water in there before adding fish? Did you try a fishless cycle? What treatment did you have to apply to your water?) Are there places the guppies could hide?
It could also be because they are afraid of the tetras. What kind of tetras are those? That is really important.
For instance the silvertip tetras, Hasemania nana has a reputation of being peaceful among tetras, but I have found that they seem to enjoy chasing and nipping guppies. Different fish behave differently with different companions.
That association of orange and more or less leaving the orange guppies alone is a real interesting observation. In behavioral studies on wild guppies, Dr. Anne Houde reported that orange and orange with a black contrast played a significant role in the response of female tp male guppies.
So whatever tetra your orange tetra is, might [lay a role in leaving in the orange guppies alone.
Of course, the blue one might just taste good.
You may be able to drop by your LFS (Live Fish Shop) and get the names of those tetras. Will they trade back?
Speaking of the shop, did they quarantine the fish? Do they do a treatment for internal parasites? If not, you might...
Or do a Google image search for Tetra (that will be a while). The host of this site is also quite a tetra enthusiast. Maybe see http://freshaquarium.about.com/cs/characins1/l/blthumbchar.htm
Miskairal asked for a run down on your tank parameters (size, temperature, filter. etc, etc.). That would be useful if you would please provide that.
All the best!
uncle scott
[ Parent ]