1500 gallons...I'm not sure how many...I thought I counted real quick 10-13 koi...plus I believe they have some comets and fantails in there too..I'm not sure how long he's had that pond...but I know he's been ponding for 20-30 years...he lost some fish years ago...His brother's pond sprung a leak and he took his fish..He told me it killed 75 of his...but I'm not sure if they were in the same pond I took pictures of or not...he lives around the corner from me and I've only talked to him a few times..he uses uv light..cleans filter daily..adds salt..vacs out the bottom..and bleaches the pond once a year....

Based on the info you have given and assuming the check valve is being placed on the suction side of your pump, If you are installing your external pump above the water line, you will need your check valve below the water line before your priming pot. If your check valve is above the water line, over time, air pockets can form in the line which can cause you to lose prime. How much below water level can you place the check valve? Your concern about the water level would be the water level dropping in the pond due to evaporation, etc. is valid if your check valve is not enough below water to compensate for different levels. This is another way air can get into the line. This is why some companies state to place the check valve one foot below water level. Your water level could drop a foot before it causes cavitation to the pump. You need to keep the run of flex hose to the pump horizontal or sloping down to the pump. You do not want to start the run horizontally and then go up and back down. At the highest point an air pocket will form at that highest level. If you can monitor your pond water level and keep the check valve always below that level by refilling the pond when it loses water through evaporation, etc, you should be fine. If you are placing the pump below water level then still keep all lines as horizontal or sloping down towards the pump as possible to prevent air pockets from forming.You would not need a check valve in this situation as the pump would be in a " flooded" situation. I hope this has helped.

I had a turtle but I only saw it once-it was probably 1/2 foot in diameter-what a treat. Something jumped into the water today as I checked things out-who knows what it was.

Thanks.....