A good pellet as a base diet, the most favoured brands on here are TOPS, roudybush and harrisons, there are other brands available which you can use but I think the general thought on here are to steer well away from the coloured ones.
My two then usually get 2 different types of veg a day and 1 piece of fruit but this does vary a bit
or sprouted seeds. I tend to only feed the sprouted seeds 2-4 times a week, 4 times when they are moulting, 2 times the rest of the time.
They get seeds as a treat for training rewards.
Harlie gets some seed as part of her main diet as it is thought just pellets aren't as suited to cockatiels as other species.
Occasionally they will also get tiny amounts of plain wholemeal toast, brown rice, wholemeal pasta or boiled egg.
As long as pellets are the main diet and not seed you should do ok
