Hmmm... I still don't know what it could be... Though, [after reading the post more thoughly] the heat may be a factor.. Dispite the fish being warm water fish, it may be hotter than they can handle. The white foam maybe a small bit of boiling. I would suggest moving the tank into shade or, at an extrem, do a 10% water change with chilled, like setting it out and puting a -few- icecubes in it. That should keep the temp down by cooling the water down so it doesn't get as hot so fast.
I'm only speculating with that idea. I don't have to worry about hot summer days because the tank's inside.
I wish I could do more.. Another idea is buying a tarp, to make a sort of tent over it. Put some foil, the reflective side out, on it.. That should block enough heat to keep it warm, but not hot.