Proof of Concept

Yes, another proof of concept. I like to test things cheaply before investing lots of dosh.

As I've mentioned previously, Hay is of the opinion that our 3 koi, in a pond which is 11m x 7m x 1.5m (average), are solely responsible for the pond turning green. This is despite me having asked numerous experts, who all agree that it's the fact the pond isn't shaded and that sunlight is the culprit.

I first bought a large, plastic tank for £20 off Facebook Market, filling it with layers of different grades of filtration media - foam plastics and fleece, which cost me £27.

Overkill for the throughput of the submersible pump I have, plus you have to removed lots of layers to see the result of the efficacy of the different strata.

I decided to buy 4 storage boxes from B&Q at £14 each, intending to stack them with the media layers in different boxes, drilling holes through the bottom of the boxes so the water would flow evenly over the media. If the idea failed, I could still use the boxes for GT6 bits.

The fleece is the most efficient remover of algae, but I can't just have that and must remove larger debris first with the foam plastic sheets. If I take the top box with the foam plastic off, I can easily inspect the fleece, which is in the next box down and clean it without disturbing the other media. The bottom box is merely a sump into which I put a hole for a flange and some piping for the water to go back into the pond.

Here's the fleece section after just half an hour:

The dark patch on the left is from where I was using the fleece in the big, black tub. Here's the water after going through the filter:

A good shade lighter than what's in the pond, but still nowhere near as clear as I'd like it. More fleece layers are called for, but the concept is proven.

The submersible pump is at the other end of the pond, so I'm drawing dirty water from one end and putting the cleaner water back at the other, using an ordinary garden hose to shift the water.

Now the pump is quite pitiful in terms of throughput, and it's not rated for continuous use, so I intend getting a more powerful one just as soon as Hay registers the fact that it works as intended, even if it wouldn't clear the pond in its current form. The Titan pump probably doesn't even stop the green getting any worse, rather than eliminating it - a faster pump would give me an edge over the sunlight, but I'd probably have to use larger diameter pipe than a garden hose.

To increase the filtration I could put a 100W UV-C lamp in yet another box, which will kill any remaining algae managing to escape the mechanical system. Commercially bought UV systems can cost £200, but I'm sure I can rig something much cheaper.

I did think of just dangling a UV light in the pond, but its efficacy would be localised without a flow in the water to move it around. Also, it can damage fish, giving them sunburn and blinding them, so that's a non-starter. Any UV system (and it's UV-C that's most effective) has to be in-line with a filtration system outside of the pond.

The filter media I'm using are called biofilters, as they become colonised by good, nitrifying bacteria. Other biofilter media, such as ceramic rings, bio balls, lava rock, sintered glass media and pumice stone provide a large surface area for beneficial bacteria to colonize and thrive, and I may add some of those to the sump box. 

These bacteria play a crucial role in the nitrogen cycle by breaking down harmful ammonia and nitrites into less harmful nitrates, indirectly helping to control algae by reducing available nutrients. Integrated into pond filters, these media ensure optimal water quality and support a healthy pond ecosystem. 

What I really need is something like a Hozelock Aquaforce 15000 or an Oaze Filtoclear 30000 system. Even then I might need a couple for a pond the size of ours. They do come up 2nd hand, but not that often, as most people don't have ponds the size of ours - it's more a swimming pool.

Another possible solution is barley straw. Barley straw controls algae in water bodies by releasing organic compounds and small amounts of hydrogen peroxide as it decomposes. These substances inhibit algae growth by creating an unfavorable environment. Placed in mesh sacks and submerged in water, barley straw offers a natural, eco-friendly, and cost-effective solution for algae control, beneficial for fish and aquatic plants, taking about 2 weeks to start acting.

We're not in a barley growing area, so I'll have to look further afield. One thing I've noticed about barley straw is that it's very inexpensive when bought in bales, but the minute the term pet bedding is added, it increases in price 100 fold - you can pay £5 for 2kg. I need about 6 kilos in total for our pond size.