Magnesium deficiency occurs frequently in potted citrus. Add 1 Tsp. of Epsom salt for every 3 gallons of soil and water to dissolve the Epsom salts. Sometimes the plant will sucker near the root graft at the base of the tree. Remove all growth below the graft. This is especially true with younger trees.
Suckers are very vigorous, thorny and are a different plant than the top of the tree.
Remove them as soon as they begin to grow. Little pruning is required. Some trees may develop branches that go in odd directions. These can be pruned anytime. The trees will develop a nice round shape naturally, without much pruning.