Hi all, I just bought a house with a small (I think 500 litre?) plastic pond in the garden. The seller was meant to take the fish but I think he only took the expensive ones (Koi he said he had?) to sell and has left at least 2 reasonable sized fish (goldfish maybe? Or orange koi??) and I spotted one smaller one. The orange ones are about 2-3 inches? I initially was planning on getting rid of the pond as I have a three year old but now he's left fish I feel guilty...so I just bought a cheap pond pump as I read online they needed the water circulated - got a pontec pondovario 750. I'm feeding them some koi food I got recommended at the garden centre. Is there anything else I need to be doing? Like a filter? I've attached a photo.