Greetings

how many gallons

Well, because of the shape of the pond I really don't know! Is there a easy
way to know?

and what are the dimensions of the pond,

It has a perimeter of 22mt

how deep is the pond

It's 60 cm deep

how old is the pond

It has 5 years

how many fish and how big

There are no more than 40 fish. Most of them have 20 cm

how often do you feed and how much at a time

Once a day, about 50grams of food

how big is the pump and filter

The pump pumps 3000 lt/h
The filter it's a beer size barrel.
It's cleaned once a day by summer, and once each 2 days in winter

is there a thick layer of goo on the bottom of the pond, when was
it last cleaned, was it ever clear

Yes there is. Well it's clear every time we clean it for about 2 months than
it gets green!