I’m sorry that you’re having this problem with what I assume is a new pond.
Obviously it’s a little difficult to be sure without knowing a bit more about the history (it sounds like you’ve had some water quality problems) the dimensions of the pond, its location, how it’s stocked and so on – but hopefully a few suggestions will help you sort things out. You’re probably going to have to do a bit of CSI-type work to figure out what’s going on.
Barley straw has a long history of use to control algae in ponds and there’s plenty of evidence to back up its effectiveness and the Centre for Aquatic Plant Management has looked into the science behind it. There are no real side-effects of using it correctly – although it can cause the water to become deoxygenated if the recommended dose rates are greatly exceeded. Between 25 and 50 grams of straw per square metre of pond surface is an ideal amount, but provided you haven’t been throwing in something like ten times as much, it seems highly unlikely that barley straw’s the problem.
Algae, Minerals and Bacteria
There are red algae – properly known as Rhytophyta – but most of these are marine species, although some kinds are found in freshwater. It doesn’t, however, sound like that is what’s affecting your pond since they tend to attach themselves to plant leaves and other suitable surfaces, growing into a distinct beard-like fuzz – they’re sometimes called “beard algae” as a result.
On balance, there seem to be two likely candidates for causing what you’re experiencing –mineral staining or bacteria. Excessive iron or manganese in the water might cause some or all of the results you’re seeing – and if your water tests previously showed elevated levels of either of these, then that would certainly be something to explore further.
However, I suspect that you may be suffering from one of the Serratia bacteria – most probably Serratia marcescens. Widely present in nature, these bacteria thrive in any wet or damp environment and spread in the air; a pond rich in phosphorus and other nutrients, and with well conditioned, un-chlorinated water unfortunately makes an ideal home. If your pond was previously very full of algae, their die-back may have released a spike of nutrients into the water and provided a bonanza opportunity for bacteria to colonise and grow.
For the Future
There are a few things you can do to help avoid these and other problems in the future. You might want to consider installing a good UV lamp and bio-filter – I’m assuming since you’re using barley straw, you don’t presently have either.
Another important aspect of algal control involves regulating the amount of light and nutrients available for algae to grow, especially in the early part of the summer. Making sure that your pond has adequate surface cover, being careful with fertilisers around the pond margin and keeping as much of the dead leaves and other organic matter out of the water as you can will all help keep the scourge of green water at bay.
Keep up the water testing too; many pond-keepers don’t really bother, or do it in a rather hap-hazard way, but as you quite rightly realise yourself, it’s the only way to be sure the most critical part of the pond environment stays in good health.
Best of luck with dealing with your problem; hopefully you’ll soon have a pond to be proud of!