9 common pond keeping mistakes and how to avoid them

  1. Too shallow

Almost everyone’s first pond is too shallow. Shallow ponds have a surface area which is too large in relation to their volume, and this presents several problems for the pond fishkeeper. Shallow ponds heat up and cool down too quickly, and satellite dish shape ponds are more prone to algae than bowl shape ponds because they become sun traps. A pond which is too shallow could freeze completely in harsh winter and even common goldfish need two feet minimum in which to overwinter properly. Herons can wade in shallow ponds, putting your fish on the menu, and water lily leaves will protrude like cocktail umbrellas, leaving the undersides of their pads vulnerable to aphid attack.

The solution is to dig a much deeper pond from day one. An 18” hole will mean maximum water depth of about 14” at its deepest. Instead, dig 30” for a true 24” water depth and if it’s too much to bear on your back use a mini digger, or hold a pond digging party and BBQ. Why not dig 24” into the ground and go up 24” above ground? All your pond fish will thank you for four feet of depth and it means you can keep Koi carp without getting sneered at too.

  1. Two ponds – two pumps

This happens all the time and I’ve seen some so-called experts fall foul of it too. You start with one pond and end up with two separate ones, but you want to connect them. In your infinite wisdom, you calculate that if you use two equal-sized pumps you can pump water to and from each one at the same time. The result? One floods and one empties. Why? Because you will never get that flow in and flow out exactly matched due to hose length, pumping at head, one pump blocking more quickly than the other and two identical pumps actually running at tiny differences in flow from day one.

The only answer is to have one pond higher than the other, put a pump in the lower one and pump out and up to the higher (header,) pond. Gravity then does the work of emptying and equalising with overflow water exiting the top pond via waterfall, cascade or wide bore exit pipe. Note that the inlet hose should be above water level so it doesn’t back-siphon in the event of a power cut (a common mistake,) or you’ll need to fit a non-return valve.

  1. Wrong fish

With the average garden pond only being about 500 gallons in volume, and say 9x6x2’ at its deepest, most “pond” fish grow way too large. A koi carp of average quality may reach 30” in the UK but may be 10” deep in the body. Ghost koi can be even broader and deeper, and a top Japanese koi could reach four feet and a foot across and deep. Sturgeon and sterlets are some of the largest freshwater fish in the world and even the diminutive species we can buy for ponds can average 3-5’ in length no problem and live for 50 years plus. Sturgeon are natural inhabitants of huge, cool, well-oxygenated rivers, not small, still, warm, weedy ponds. Golden orfe and blue orfe regularly attain two feet in length, are keen jumpers when young and have a high oxygen requirement. A heatwave and still night and the orfe are always first to succumb.

 Tench grow large and you won’t see the green ones. Rudd, Roach and Bream need lakes and ponds, chub, dace and barbel need rivers and trout, although available need cool, well-oxygenated water, are predatory, territorial and will jump out, guaranteed, if warm water and lack of oxygen don’t get to them first.

The perfect fish for your 9x6x2’ pond are common goldfish including those fetching Canary yellow ones, comets and red and white sarasa comets, calico comets, Bristol and London shubunkins. They will grow but not outgrow, are active, colourful, easy to keep, cold and hot weather hardy, and easy to breed.

If you want koi then build for koi from the start. Massive mechanical and biological filtration, four feet water depth minimum and ideally but 12x9x4’ minimum, but larger and as deep as you can afford.

  1. Too many plants

Pond plants are ideal for fish ponds in so many ways. They provide shelter for fish and shade from the sun, which helps fight algae. They fight algae by using excess nutrients as a food source and some outcompete algae by fighting them with allelopathic chemicals. They can be planted to look either ornamental or natural and fish and amphibians use them to spawn in. But if you have too many they can strip a pond of oxygen at night, diminish swimming space and all those dead leaves can pollute in their own right.

Plant ponds with a mixture of marginal, oxygenating and deep water plant species but regularly cut them back so that they are never covering more than 50% of the water surface. Remove, cut back and re-pot every few years and take regular photos as you’ll be surprised how much they can grow and take over.  

  1. The pump is too small

I dealt with this many, many times in my time in aquatic retail. A tiny fountain pump of say 700lph gets reduced by 50% and you sell half a pallet of them on a weekend. But unless it is a pebble pool you have, not a pond, that pump will be too small. 700lph is at zero head, so attach a hose to it and try to get it to pump uphill and that will be greatly reduced over just a few feet. A small pump will also have a tiny inlet strainer, meaning it will clog quickly and if not maintained daily in summer in a pond stocked with plants and fish, it will block or stop, just when you don’t want it to – on the hot days or when you’re on holiday.

For a decent waterfall, you will need 4000lph, and if you split it by operating a fountain and waterfall you will need double that for an effective display. There are fountain pumps and there are solids handling filter pumps, and few are good at doing both jobs. For a filter you want a large open cage to suck in dirt and debris and take it off to the filter. For a fountain you need a fine, low-velocity cage will protect the small holes in the fountainhead from blocking. The best solution for a pond with both filter and fountain is two separate, purpose-built pumps.

  1. Not netting fish

Pond cover nets look rubbish but they are so important for ponds containing fish. Orfe and trout jump out, but so do newly introduced fish, scared fish and those irritated by skin parasites. Expensive koi are known for jumping out the night they are introduced to a pond and what about the heron? Nets keep leaves out too.

Arm yourself with a pond cover net and use it for at least a week after introducing fish. If you have a square or rectangular pond with edging you can stretch the netting over removable frames and you can then use them at night and remove them when you are in the garden viewing the fish. Better safe than sorry.

  1. Not medicating properly

Pond fish come to the Uk from all over the world and are intensively farmed in their millions. Any high number of captive fish is perfect for parasites, and stress of capture and transportation means run-down fish which are susceptible to disease. Mix them all together in the wholesalers, retailers and then your pond and it’s your responsibility to treat any new fish as carrying a disease. First of all quarantine the fish in a separate holding vat with filtered, stable water conditions.

Medicate the fish with a pond anti-parasite treatment and watch it for a few weeks in the vat, checking it is swimming and behaving as it should and that its fins aren’t clamped. A few weeks of healthy fish and you should be ok. Better still though is to get an expert to conduct a skin scrape and look at it under a microscope. If a parasite is detected it can be identified and then the best treatment used on it. Then scrapes can be taken again and again until no more parasites are detected.

      8. Liner exposed

This is more personal taste than life or death, but so many first time DIY ponds result in a large area of visible black liner on show which really detracts from the overall look. A pond edge should look neat and angular and man-made, like on a brick-built raised pond, or natural, like with a ground-level one.

When digging the pond make sure that the edge is clean and neat, and that the flagstones, turf or sleepers come up to that liner at 90 degrees and then neatly edge the water below. 12” of exposed, creased black liner on a gradual incline will look rubbish, shatter any illusion of either ornamental or natural design and is also at risk from cat, dog and heron claws. Plan properly and cover it up, or bite the bullet and get a landscaper to build the pond for you. Ponds aren’t ever short term things so it’s important to get their design and look right, for you and your fish.

  1. Pebbles

Clear water over rounded pebbles with dappled sunshine raises a primaeval attraction to us and without exception we find it exquisitely beautiful and natural, and calming. But how it may look long term in a stream or river is very different to re-creating that look and habitat in a pond. Pebbles look clean in a fast-flowing river because they are constantly being washed, moved and eroded. This prevents debris from accumulating and algae and biofilms from growing. Fast-flowing rivers also tend to be naturally low in nutrients and pollutants which again helps to prevent algae growth.

Put a bed of pebbles into a pond however and you create a natural mechanical filter, where all the debris and uneaten fish food drifts down and settles between them. This causes algae and water quality issues. Couple that with the fact that the stones aren’t being tumbled, eroded and cleaned on a daily basis and those smooth, bright clean stone surfaces quickly become colonised with nuisance algae like blanketweed and the whole look is spoiled. Use pebbles around a pond above the waterline, but don’t put them in the water where the fish are, in number, as they just collect dirt and go green with algae.  


Jeremy Gay

Author of three fishkeeping books and lifelong fishkeeper. Experience includes editor of Practical Fishkeeping magazine, editor of Pet Product Marketing magazine, multi award- winning livestock manager and aquatic store manager.