Sounds like whitespot... are they itching, scraping themselves on substrate etc?
Some people like to nip whitespot naturally, other's use meds..
I am one for natural before anything else, raise temp to 28-29 degrees, extra airation and a bit of tonic salt... raised temp speeds up whitespot cycle..
Cold water changes can definately cause it, especially when you have said "large water change" to. Too much of a shock to fishes system... some are more sensitive to this than others.