I'm not sure a problem has been identified. Going back to the photograph, the Comfrey is pretty much growing in the pond in the pebbled bog area. As such it's acting as a weak biological filter and will be pulling nutrients out of the water as it grows. The Comfrey leaves will only be a problem if they are allowed to rot.
The rat-tailed maggot could survive in polluted or stagnant water, but it's not a definite indication of either.
I've already asked the question, and it's not been answered;
What is the Comfrey growing in?
If it's regular compost or garden soil and it's in contact with the pond water then this is more likely to be a problem than the Comfrey itself.
It's also not been clarified how old this pond is. If this pond is less than two years old, then the best thing it could be given to encourage wildlife is time/patience. It will take at least a year for the water to get itself in balance whether the water used to fill it came from a water butt or the tap.
If it was built after Autumn last year, then it will have been missed this year by the frogs for breeding. And in any case, it would be too new to reliably support tadpoles for at least a year.
An update photo would be useful, the photos posted so far show a rather bleak and unwelcoming pond from frogs point of view. They like somewhere to shelter where they can feel comfortable and hidden from predators. Your marsh marigolds will produce this in a few seasons, but think about some floating plants and introducing a bit more "straggliness" around the margins (perhaps some miniature reedmace such as Typha minima). With leaves that overhang the edge and provide shelter all the way from water-to-soil. Hostas are good. And don't worry about slugs, where there are slugs and water there will be frogs in time. I lose a little early spring vegetation to slug/snail damage, but once the pond warms up and things get going (and in East Anglia this is well into May before this happens) the frogs soon reduce the pest populations without pellets and the summer leaves are safe.
Unless you're going to tell us that the pond is actually five years old.. I think most of the problems aren't really problems and will resolve themselves with time. We're not even in June yet, and ponds are really late-spring/early-summer gardening. The warm spring has perhaps created expectations a little early this year.
Don't buy oxygenator weed.. use your local Freecycle/Freegle group.. come high summer I'm usually giving it away by the bucketful locally, along with divisions/seedlings of the marginals and even the excess goldfish that have bred in the pond.
Skimming surface gunge and topping up with rain water is a good idea, but only if the rain water is reasonably fresh. If it's been standing a while, if there's debris in the water butt or if it has an odour.. then don't use it for the pond. Locally we've had so little rain that the rainwater we've got left in the butts is too old, contaminated and oxygen depleted to use. If you do have fresh rainwater, give it a good oxygenation - use a bucket to lift and pour back water into the butt so that it pours fast and splashes loudly. By this sort of violent disturbance you'll get some air back into water that's been sitting a few days.
If the worst you do is add tapwater, it's not the end of the world. It will cause a minor algae bloom.. but nothing compared to the bloom you'll get using the wrong compost for aquatic, marginal or bog planting.. if you do use tapwater, use a hose and make the most of a moderate jet of water to disturb the water already in the pond to get some air into it. Unless we get a torrential downpour in the next couple of weeks I'll almost certainly be topping up my pond with tapwater.
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July 06, 2012, 10:59