A lava rock filter isn't going to cut it. With a pond you really want to have a prefilter before your biological filtration. Lava rock has a tendency to plug up after a year or so. You will have to take it out and spray off and that will kill off your beneficial bacteria. Get a 154 or 30 gallon drum and make a moving bed filter that uses k1 kaldnes. Get an air pump that pumps A MINIMUM of 25 liters per minute. Also get yourself a real pump. Get an evolution es1500 with 2 inch unions. You also should get a retrofit bottom drain that sits on the bottom. You could run the pipe coming from the drain through a bulkhead fitting through the side of the stock tank, or your could run the pipe directly out of the stock tank. Running the pipe over the edge probably won't look as nice as running it through the side using a bulkhead fitting, but both will work. With your current setup, you are going to have to drain it and clean it out every year. With a bottom drain setup, you will not have to do that. What size is your pipe? I am guessing it is too small. If it is any smaller than 2 inches, leaves and debris are going to get caught in your pipe. You also need to have a priming pot on your pump. I built my pond without a priming pot and leaves got caught in the impeller of the pump. I have a priming pot that i am going to add pretty soon. I f your filtration is not good enough, you are going to get an algae bloom. If your filter is not processing enough ammonia, algae is going to grow and do that. You do not want green water. The algae will die off once you have a good enough filtration system, but the rotting algae that caused the water to turn green is going to cause an ammonia spike. You need a better filtration system, or you are going to get an algae bloom.
