Hi Sand and welcome, ;) Small Pond, how small is small? ??? (Never fed? ???) If there were no plants what were the fish eating? ??? Algae? ??? By adding food you have upset the balance, although that should right itself in time providing you keep the filter running permanently, the addition of plants will help in the long term, but not knowing the size and how many plants going in, will they have room to live in comfort? ???
Keep on top of cleaning the pump and filter, (but don’t clean the bottom medium), whilst feeding and include small 15/25% water changes too, especially if the weather is hot, :o (Don’t forget to de-chlorinate the new water).
Are you glad you asked? ;D