Keeping Fish in a Pool – can the pool filter stay connected?

This filter was unplugged when fish were in the pool.

This article has been authored by the owners of this site. No input or information has been generated by any AI systems.

When we decided to keep fish in our in-ground concrete swimming pool, we faced the problem of what to do with the existing filter system. There was very little information on the topic available back then so we had to think about the potential issues and see if we could live with any problems that could surface.

This article is about how we approached our system, and how the information we picked up might help anyone who is considering a similar undertaking.

Can a pool filter be used with fish in the pool?

We don’t recommend you use the pool filter when you have fish in the pool. The normal pool filter is designed to service a swimming pool, not a fish pond.

Our filter system is a typical sand filter with a high-pressure pump that pushes the water through the sand and returns it back to the pool. In practice, these filters are designed for relatively clean water. Your fishpond/pool will have algae, fish poo, maybe some water plants, and a myriad of water insects.

If you decide to use the existing filter, be prepared for a lot of filter back-washing and water replacement, because the pond will overwhelm the filter in time. It also removes many of the beneficial life-forms that make up a healthy pond system.

We never used the pool filter while fish were in the pool. We did have other filters in place that are discussed in this article.

The sand filter bulb will hold water if the pump is unplugged for years.

The next issue with using the existing pool filter is the volume of water that is moved by the pump. We wanted the pool to evolve into a semi-natural ecosystem, and the pool pump would limit this via too much water flow.

The only scenario that we can think of where you want the strong water movement is in a dedicated fish farming system, and water quality is paramount to the success of that operation. Ours was a simple backyard system where we could grow protein in a beneficial way that suited our lifestyle at that time. This article gives a big picture view of what we did.

What did we do with the filtration system?

We left the complete filtration system in place, with the power cord removed from the pump, and all pipe work and plumbing was left in place and the pipe ends left open. We found that small insects like dragonfly larvae used the open return pipes as shelter. Leaving the system in place was a net benefit in the big picture.

Will the filter harm the fish?

This is something we were cautious about, as our pool had been converted to a saltwater chlorine pool, and the chemicals that were used were not great to have around fish.

We knew that the sand filter would hold a residual amount of the old pool water once we exchanged the pool water with fresh water, and we decided with the pump disconnected there would be little risk of harm to the fish from that contained water once we introduced them to the pool/pond. This turned out to be correct.

The benefits of leaving the filter in place.

The first consideration is the cost to remove it. In our case, the return pipes would need to be blocked or removed and the concrete wall patched somehow. The sand filter would then need to be removed and it holds a lot of wet sand. Your system may be different.

The pool overflow piping would need to stay for the times when heavy rains overfill the pool/pond, but this section of plumbing is related to the skimmer box and would be very difficult to separate from the rest without careful cutting of concrete, and a lot of excavation. Simply leaving it in place avoids all of the above and is far cheaper.

Removing the skimmer box when we put fish in the pool was not considered

The next benefit, and in our opinion the best, is the ability to return the pond back to a swimming pool if desired. This is the path we have taken, and we have an article planned around this process to help others make the change, both to put fish in, and then to change back once the urge to grow fish has been cured.

The initial decision to throw fish into your pool can be easier to digest if you know that there is a chance of reversing the process in a few years’ time.

Will the pool filter be damaged if it is just sitting?

The main infrastructure should be fine, and your specific situation will determine what you need to do, should you wish to ever reverse the process. We had our filter turned off for several years, and the following is what we found.

We found that the sand within the filter bulb was fine, and all the pipe work was still functional. The only negative was the pump was in need of replacement, but it was several years old, maybe even ten years, already when we unplugged it to put the fish in.

It would have needed to be replaced in due course anyway, so we replaced it when we reversed the process back to a swimming pool. Keep in mind that the water in the pool was saltwater chlorine, so it is a bit corrosive to start with.

We found that the pipes that moved the pool water around were ok, with no leaks and no damage. In fact, it was quite an easy task to get the pond back to a pool.


To finish up, it is ok to leave the pool filter connected and unplugged. The modern filtration systems are robust and designed for longevity, and not using it will cause minimal harm to the filter system, generally speaking.

Article by Tim Blanch for He is a qualified Permaculture designer.