If the maintenance as described for March have not been carried out yet, it is high time now. Carefully inspect for leakage outside your pond.
To obtain a prolonged luxurious growth of pond plants, certain minerals are necessary. Especially the tracer element iron is an important growth stimulating mineral. Naturally it practically does not occur in pond water. Therefore add an iron fertilization.
This is the time to plant new water lilies and to supply existing water lilies with new substrate. We advise you to use a nutrient soil that has been composed especially for water lilies. One that contains blue loam and dried fenlland peat.
Now you can propagate and transplant marsh plants. Use suitable plant baskets and plant soil. You may also plant them alternatively in those nice floating plant islands for beautifully grown over floating islands in your pond.
End April the filter and the pump have to be started up again. You should decide yourself whether you want to switch on the UV-C lamp too. Not only plants start growing well, also algal growth may cause difficulties now. Strong growth of fibred algae will check the growth of oxygen producing plants. It is recommended to apply a course of anti-algae treatment or to install an electronic algae remover in order to control these awkward algae.
Observe your fish well and inspect them for any symptoms of disease. If any spots and ulcers are present, a bath treatment with some special medicine can be useful. At water temperatures over 54°F (about 12°C) fish can be fed ad libitum. Your fish have to regain their strength again. Use therefore high-grade feed.