praziquantel is a safe drug and can be administered into the main tank. but do water change.
first you must know what you want to use it for. it is used to remove intestinal parasites and body flukes and unless you suspect your fish have any of those, you do not need to use it.
for treatment of internal parasites most commonly found in angels and wrasses, soak food in prazi for 10 mins and feed the fish. continue treatment for 1 week or more if need be.
prazi is very safe so overdosing is not a problem.
for treatment of flukes, either prazi the fish or just do FW dip. i prefer FW dip as it's more useful. i personally tried bathing them in prazi for flukes but it did nothing and needs to be done for long periods of time.
as for safety with xenias, it's not anecdotal and no hard and fast rule. people in reef central report having problems with xenias while others don't. i personally dosed it in my main tank with xenias and nothing happened.
i did have 2 fish casualties though. so like any other drug, i would reccomend dosing it in a quarantine tank if you must, but for flukes only. as a rule of thumb i generally don't dose anything in the main tank now.
if treating for internal parasites, dosing in water will not help. you must soak the food in it and feed.