Hi Koko, it's an open pond, there is no filtration, it's natural spring water, i use the water for watering the plants and refill it. From the spring water. I have 3 koi carp in the pond about 20cm long and about 15 goldfish ranging from 5 to 10 cm, there were a few plants but they gave been eaten, there are a few rocks in the pond but not much else. It was originally just a water store in case of fire or drought, I was watering directly from the water tank which is pumped up From the spring and then decided to use Pond water as a bio Fertilizer for the plants I am growing, so I water some with the pond water and then top up from the tank. It could be the dog has been in there but I didn't think it was. I will check the picture and see if it happens again and try to do a more thorough checking of the fish. Thanks Ellis