I did a filter pit this year with bottom drain. My pond is 3,000 gallons (4 feet deep at BD. Got better filtration by making my own DIY spraybar and microscreen in a 55 gallon barrel. Works like a charm! I use a Laguna filter falls for my bio, and it is filled with 1/2" plastic strapping for bio. All summer and fall it worked great!
Ethan
The pond

"Glitter", my ginrin sanke 17"

my two step Kohaku 12"

My odd looking matsuba that no one can classify 15"

my doitsu kikokuryu 12"

my armor scaled yellow ogon 12"

24" butterfly sanke "Speedo"

15" sanke, "SPOT"

24" shiro bekko butterfly "sizzle"

22" butterfly mutt "Striker"