i have 9 rosy red minnows in a 15 gallon tank and every now and then i see them at the top of the water, almost look like theyre gasping for air. what are they doing and why??
If you have plants...
in the day time: plants convert carbon dioxide into oxygen (to breath)
at night time: plants convert oxygen into carbon dioxide (= less stuff to breath)
There is more oxygen towards the top of the tank so they go there.
So a combination of plants and bad aeration (by your filter) could be doing it.
There trying to tell you to put them in my snappers tank!!!!!!JKJKJK..........could be a really low level of dissolved o2 in the water.....increase water movement,add an airstone and do an immediate 10% WC,followed buy hourly WC's until they regain their vigor.
This tank is not cycled - the minnows are being used to do so. Their behaviour would indicate to me, most likely, that you have ammonia/nitrite build up beyond that which the fish can tolerate. Test with a good liquid drop test kit - API Master Test Kit is great. If parameters are above .25 ppm for ammonia or nitrite do an immediate water change to get it below - change as much as is necessary to do this.
You may as well check your nitrate too although it is almost certainly not a problem - in the long term you want to keep this below 20 ppm.
If you have 0 ammonia, 0 nitrite and are sure you are doing the test right, it is time to look at surface agitation/oxygenation...but for now I'm going to say the simplest answer is probably correct...its a nitrogen cycle problem.