If you have a bare bottom tank it will be the easiest to see if your fish is constipated.
The easiest telltale signs will be: (In my personal experience at least!)
There will be no fish waste in the bottom of the tank. (no fish poop)
The fish will begin to have a rounded belly which often turns white-ish.
Here is a link from Nippyfish on constipation:
Here is a link to walgreens epsom salts
If you look at the ingredients you will see:
Magnesium Sulfate USP (Heptahydrate) - 100 %Saline Laxative
Sorry to hear you lost your fish Brettwashere. :(