If you have water plants or water lilies, it is recommendable to renew the nutritive soil. Put some pond soil in a larger plant basket and the lilies will growth well again.
Remove bottom sludge already in March by adding Vincia Sludge Cleaner to the pond water. Moreover it is important to check the hardness of the water using the Pond Test Set. The general hardness should be at least 8° GH. A too low value can be raised with GH-Plus. In order to destroy floating algae and germs activate the UV-C filter already now. Please do not forget to replace the old UV-C lamp.
Carefully inspect your fish for any symptoms of disease. Cotton wool-like spots, ulcers or swinging motions indicate parasitic disorders. Feed the fish moderately now, because the fish are becoming active at rising water temperatures. Bring the fish in top condition again. After the cold winter months their condition has reduced strongly.