Used to have a 1000 gallon pond. One turtle RES and mix of koi and gold fish. Used a simple drum filter with appropriate sized pump. Always clean except before the plants took hold. Then I had a fight with algae as it was in sun all day.no doubt my opinion is based on anecdotal evidence .... i use to monitor my water parameters , it's been a decade or two since i've done that ..... i just found what works and assume it will always work ....... a pond where i live , especially a small pond needs a proportionally large efficient nitrification process as the water temperature is under 65 degrees most of the year , the nitrification process is at best 50% , probably more like 30-40% efficient .......... very different in a tropical fish aquarium where at 80 degrees it's probably 80% efficient ........ i have gone along with the assumption if it works in a 250 gallon cold water pond with four 8-9" adult turtles , it must work anywhere ........ we've had 40-50 degree nights this week , i don't monitor the pond temperatures very often anymore either , i bought one of those test kits today , should get it wednesday , be interesting to see what i think but really don't know ....... when i vaccuum the ponds it draws the water down , when i replace the water it stirs the "sludge" from under the ug filter , i'll try to test the water after i do that ...... should be a good indication as the water in the ponds is pretty cool right now , the nitrification process is pretty inefficient right now .....