One of the problems with some owners converting to an all pellet diet is that it provides too high a dose of calcium. Testing has found that a Pellets only & Poultry food diet causes an increase in calcium deposits in the kidneys which  then causes renal failure & death. When switched to a mixed seed diet the deaths stopped.

As is always recommended in the FAQ's on this site, all budgies need a healthy diet which includes 30% seed, 30% pellets, 30% fresh foods and 10% treats such as honey sticks, millet etc. Their water should be changed daily to avoid build up of bacteria and they should always have access to a cuttlefish bone (or calcium bell or block) and an iodine block. This gives budgerigars access to all of their daily needs.