How to clean a Koi pond without killing it

I always cringe when people tell me about how they drained and power sprayed their pond, and now the water has turned green again.  Sure, you may have crystal clear water with sparkling rocks for a few days, but before you know it, stuff will start to grow on those pretty rocks, and the water will turn green once again! 

The first thing to decide is whether you want a fairly clean pond that is full of life or a perfectly clean pond that is virtually sterile: you can’t have both, unfortunately. If you choose the latter, then you need to set it up like a swimming pool using chlorine or a similar product in order to prevent life from growing. But if you want fish or plants in your pond, this will not work out so well.

If you opt for the fairly clean pond that is full of life, you are going to have to make a few concessions.  First, your pretty rocks are not going to stay sparkling clean.  Remember, this is going to be a living system, and as such, life is going to grow on EVERYTHING!  Second, you are not going to be able to feed your fish at levels like they are training for a triathlon.  When fish eat, they poop, and that fertilizes the pond, which leads to algae growth.  Third, you are not going to be able to power spray your pond anymore. Power spraying strips the biofilm—a thin layer of beneficial bacteria—off of the rocks.  It’s a subject for another day, but the living bacteria in a pond is what keeps the water clean.  The last thing you want to do in a “living” pond is kill it all off.     

Back to cleaning a pond.  Now that we established that you want to prevent harming beneficial organisms in the pond, the question is how to clean it without killing it.  Sometimes, a pond gets filled with leaves or other debris, and you have no choice but to drain it down and clean it out.  If you do so, do it quickly. If at all possible, refill with water the same day before all the rocks have dried out.  Even on hot days when rocks exposed to the sun dry out quickly, you will notice that the areas beneath and between rocks stay wet and cool.  This allows surviving microbes to recolonize the areas that did dry out and will make the pond bounce back quicker. 

Now let’s talk about what to do, or what not to do, with the fish during a cleanout.  First of all, we recommend that you never mess with cleaning your pond just before, during or after winter.  Pond fish, especially Koi, cannot recharge their batteries during winter, so if you stress them out bad things can happen.  Also, if you do a thorough pond cleanout where you have to remove your fish, you need to think it thru ahead of time on how you can do it without temperature shocking the fish in the process.  What you need to do is pump pond water into a kiddy pool (or a similar structure), move the fish in, turn on an aerator, cover them to keep sun & predators out and the fish in (they will jump), and then clean and refill the pond as quick as possible.  However, if you refill with 50 degree well water and the fish are in 75 degree water in the kiddy pool, you have a problem.  NEVER change their temperature by more than 10 degrees in 24 hours, preferably 5 degrees, and change their temperature gradually through acclimating.  Therefore, the best time to do this is in the spring after the fish are actively feeding and have recovered from winter or late summer/early fall when the water has cooled a touch, but well before cold weather sets in.

If your pond is not chock-full with debris, you can clean the pond without removing the fish!  This is my favorite approach of more frequent, partial cleanups.  To facilitate easy cleaning, I like to maintain bare liner in the deepest portion of the pond.  There are different opinions as to the benefits of gravel on a pond bottom.  My personal opinion is that it does more harm than good by making it difficult to clean.  By maintaining bare liner in the deepest portion, it allows you to go after that waste more easily without having to drain down the whole pond.  Because it is the deepest spot, gravity will bring most, but not all, of the waste down to this area.  Remember, this is a partial cleanup.  We are not trying to get every last bit of debris out of the pond, so don’t worry about it!

Finally, how to get the debris out of that deepest portion of the pond.  If the debris is large, like leaves, there is no substitute for getting in there with a fish net and going at it.  But this doesn’t work for the black silt that builds up.  There are various types of pond vacuums on the market that work well for small partial cleanups.  The biggest problem with the shop vac style devices is that you have to go thru filling and draining cycles with most types.  My personal preference is to place a Cleanout Pump in the deepest area along with a hose that runs out into your lawn.  Depending on what sort of pump you have, you may need to put it in a Pump Cage to prevent fish from being sucked in.  We sell these, or if you are crafty, you can build your own.  Next, plug the pump in and carefully sweep the silt towards the pump.  Essentially, you have to liquify the silt, and then the pump will remove it.  Once again, you don’t want to get hung up on removing all the silt.  Just focus on getting the majority, and then let the rest settle for next time.

It is ironic to think that cleaning a pond can harm it.  But it is definitely the truth if you go about it the wrong way.  Now if I could just convince my wife of that next time she asks me to clean up around the house…


You can find products discussed in this post on the Pond Cleaning Supplies page.