How long pond fish survive without a filter depends on a few factors, they should be ok for a day or two without a filter, in certain conditions they could live longer.
For instance, if you have a large deep pond with just a few fish, it is possible the fish would be able to live without a filter, but if your pond is well stocked with fish you will have problems, especially if you have Koi.
If the water is clean your fish will live longer without a filter, but as soon as the water starts to get dirty your fish will start to suffer.
Without a filter your pond will get contaminated with fish waste (poo) uneaten food and rotting plants, this will produce ammonia and nitrates which pollutes the pond water, all of this will have an effect on your fish and your pond plants, if nothing is done your fish and plants will die.
The above scenarios are for established fish-ponds that have suffered a filter failure, but what if you want to build a pond from scratch without a filter, or you want to convert an established pond to run without a filter, can it be done.
The answer is yes it can be done but you will have to follow some simple rules.
First you need to have a pond that is large enough to accommodate your fish, and when I say fish I’m talking about goldfish rather than Koi, I’m afraid Koi will need a filtered pond if they are to survive.
You can not overstock your pond, the more fish you add the more waste they will produce and before long your water will become toxic.
So the first rule is to add just a small amount of goldfish to your pond, if this is a small pond just add a couple of fish and see how you get on, if it’s going to be a large pond then you could get away with more fish but remember do not overstock.
Secondly do not overfeed your fish, you might think you are feeding the right amount but you just need to feed them enough to survive.
If you do overfeed your fish any uneaten food will fall to the bottom of the pond and decay, this will release harmful ammonia and nitrates into the water which in time will kill your fish.
The next thing you need to think about is the water, this needs to be cleaned manually, normally the filter would do this job.
You can safely do a 30% water change without affecting your fish, just remove 30% of water from your pond and replace it with dechlorinated water.
You need to remove all the solids that have built up at the bottom of the pond, this will be fish waste, uneaten food, rotting plants and algae, you can do this manually with your hands or a net, there are also pond vacuums that will hep you do the job.
Pond plants are a must if you’re not going to use a filter, submerged plants will help to remove waste products from the water while floating plants will shade the pond water from the sunlight, this will help control the growth of algae.
Lastly is additives, these can help you control your pond water and keep it clean, there are many special additives that you can use, one we would recommend is the type that has beneficial bacteria, these will help to break down the ammonia and nitrates that are dangerous to your fish.
If you do all the above it is possible have a pond where fish can survive without the need of a filter.