You most likely stirred ALOT of debris up during the change and caused a mini cycle or at the least a spike in ammonia, trates etc.. Just keep up on water changes at least 50% and get that water tested. Maybe invest in your own kit there cheap online. API master test kit.
I'm willing to bet you just stirred up a lot of crap. It will all settle and you'll be ok. Sorry for the 2 losses though.
I lost a fish when I switched over to sand. It can be stressful to a fish. Did you take the fish out? What kind of fish?
Have you cleaned your filtration? It good be loaded with debris and sand.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley