I had a indoor pond with sides made of wood and black liner. not sure if this is similar to your setup. My prefence however on any tank is undergravel filters are not of my choice. all big waist just sits on top of filteration creating nitrates. I had nothing but problems. I swiched to canister filters for the large tanks and pond and vaccumed the gravel with a python during water changes. Never had any problems since. not every method works for everyone. You'll have to find what works for you.
