It’s always a bit disheartening when a crystal clear pond seems to go wrong, but don’t despair – I don’t think you’ve got too much to worry about!
The Dreaded “Green”
Your pond’s still fairly young and you made it at the back-end of the summer last year, so I suspect it probably hasn’t fully matured and settled down yet. This can take a surprising amount of time, depending on pond size, depth and the weather – amongst other things.
You’ve added a biofilter and a UV lamp to your pond, so it sounds like you know all you need to know about the reasons for green water and how to cure it; I think you’ll just have to let it do its job over the next few weeks and hopefully, now the lamp is fully functional, you should find the problem clears up.
Check how much of your pond surface is covered by plants – too much light reaching the water at this time of the year can be a particular problem, since algae grow so very much quicker than other plants. If plant cover is too little – around a half, or even as much as two-thirds being about right – then you have the perfect excuse to enjoy yourself picking some new varieties at your local garden centre!
By the sound of it, the froth is probably linked to cleaning out the filter – as well as the water not yet being fully matured. When you cleaned the filter elements, you probably (and unavoidably) stirred up all manner of sediment and attached biofilm material and I’m guessing that this is the at the bottom of the problem with the froth.
Protein-containing solutions froth easily and since you’re constantly agitating the water with the return from the filter, I suspect that this is the reason for the persistent froth you’re seeing at that point. The good news is that it’s actually not that uncommon and if this is the case, it should sort itself out as the filter bacteria re-establish themselves and begin to do a thorough clean of the water. If it seems to be going on too long, or if you’re not convinced, try testing the water – I’d put money on finding that the dissolved organic content is too high.
If there are any trees or other sources of organic material around your pond it might also be a good idea to take suitable measures to ensure that leaves, grass clippings and so on don’t make their way into the water, or this problem could recur – especially if the pond is a relatively small one.
Sorry you’ve got a problem, but if you keep running the filter/UV combination and can be patient, I suspect it will clear up on its own in good time. Enjoy your pond!