Avoid giving your dog any food that contains added salt, e.g. a bit of your takeaway meal or processed ready meal. Salt is sodium chloride and the sodium part of salt causes fluid retention whether it is people or whether it is dogs that take steroids. Make sure the dog's food is unsalted.
See my website http://www.wildeaboutsteroids.co.uk/ (The site does not sell anything and has no banners or sponsors or adverts - just helpful information.)
Read my Mensa article on Obesity and the Salt Connectionprescribed steroids and HRT