WHen you change the water are you dumping buckets in the tank in anyway that could be described as aggressively? For example: Does the substrate blow out of the way as you add your new water?
They will do fine on 2 changes a week but the stress of interrupting their living routine with algae scrubbers and water change tools etc. isn't worth it unless you're growing them out. How much water are you changing each time?
A twice a week (Again not necessary) should not exceed 15% 20% imo.
A couple things. The mouth open thing accompanied with aimless swimming is a sign that the dissolved oxygen of the new water has been expressed. This happens when the water sits without agitation for some time before adding it to the tank (Some folks sit their water to express chlorine, but the same folks usually or at least they should aerate or somehow agitate their change water while it sits.
secondly ammonia toxicity (as well as nitrite but I'm guessing you've got nitrites under control due to the fact that the others aren't acting stressed.) will cause them to gasp, opening their mouthes unnaturally, and struggle to compose themselves in general. If your local water is treated with chloramine, depending on the water conditioner your using to dissolve chlorine, you may be using a product which dissolves chlorine but does not handle chloramines (Basically ammonia and bleach). Or you may not be dosing it properly for chloramines.
Due to the fact that you've got Comps in their which are fine during and after changes I'm guessing that these Calvus which are showing signs of stress are juveniles and/or not sexually mature and/or as big in general as the comps in the same setup?
Calvus are a bit more delicate than comps (many comps are found near river tributaries which provide less stable (but still very clean) water conditions than those found at the depths and locations which Calvus are genetically accustomed to. However the largest difference in aquarium stress between the two species, and even within the same species, will be seen in juvenile Calvus. While large Calvus are a bit touchy to WC's juveniles are VERY.
The last things I can think of is perhaps your using a digital thermometer? All I know is that I have 2 digital thermometers, identical in brand and model, both with fresh batteries and yet one read 2 degrees high and the other reads 3 degrees low. That is based upon the mercury thermometer reading.
Drops in temp of 3 or more degrees can kill the young ones especially if you add sit water, or pour it on their heads or chloramines or any combo of the above.
The other thing that's dangerous is drops in PH. you may have unstable swings from water treatment, you may not be buffering with filtration elements or any number of those relative things. The bigger fish won't react as much to low ph swings (within reason. If you start throwing the water .5 or more lower than they're used to, you'll probably have a stinky fish room before too long
) but it'll kill the juvi calvus without much noticeable swing. If you must swing a ph it is much safer to swing up although both should be avoided.
Get yourself a "high range ph test" for freshwater. make sure it is a drop and vile test and not a dip strip test as they're not as accurate. If PH swings are your problem, your tap water will, most likely read lower than your tank water. make sure you sit and aerate your test tapwater before testing it, at least for an hour. because gasses and other buildup from pipes may skew your results. if it is swinging you can solve this problem by adding buffer for which their are pro, pre-formulated salts, or you can go homemade which is cheaper and equally effective imo. just check the forum for "homemade buffer recipe" for proper mix. buffering, where applicable, should make your water less susceptible to water changes at the chemical level so you'll effectively be presenting new water that is only different in the sense that it is cleaner since the buffers you've put in the water will absorb the acids which lower PH amongst other things.
I would check it in this order:
1. Sitting water. Do you sit your water un-agitated for longer than 30 minutes before adding it to the aquarium? Are you quickly dumping new water in? IF so stop it.
2.Temperature double check. Make sure all thermometers are accurate.
3.Chloramines. check local water supply on the web to see what they use to treat your water and make sure your chlorine dissolving agent is appropriate for your findings.
4.Get a drip test and compare ph. Buffer if it swings. might as well get an ammonia test too (also drip test) along with nitrate, nitrite just to be safe.
if you don't solve it with these tips. lemme know and we can discuss the exact parameters of your setup and get to the bottom of it. hope this helps and sorry for an extended response. I love talking fish and I type real fastso...
