read some negative experiences with under gravel filters .... myself i've never found them anything but helpful ...... since i haven't checked my waters parameters in a long long long time , i just assumed what i did worked before and would work forever ...... i got one of those water test kits , not sure of their accuracy ........ i checked a pond with a large under gravel filter today , the pond is 3-4yrs old , maybe older , it's a pond i made deeper due to a really bad winter freeze we had...... i'm not advocating anything i do , i do what has worked for me ...... this pond has never been cleaned , aside from removal of leaves after the winters , never done a water change , i've added water when the rain doesn't keep it full .... i've never opened the outside filter until today , amazed at how clean it was , obviously due to the water having to pass through the gravel before it gets there ..... i've never tried to remove the sludge from under the gravel ,, i do know how to remove that sludge , just never done it because of the emerald shiners that live in this pond ..... 4 adult wood turtles 8-9" live in this 200-300 gallon pond , no plants for the last 2yrs as where i get my plants is closed to anything but mail orders due to covid , here's what i got
ammonia
nirite
nitrate
ph
ammonia
nirite
nitrate
ph