Hi Steve, a pergola's a nice touch and something I wish I'd done too. I'm not sure it'll help with water clarity though, other than preventing leaves and stuff dropping in but what it may help with is providing some shade which is good. Providing shade will help keep your water temperature a bit more stable and also minimise unwanted algae growth to a degree. It has other benefits too, like helping to reduce the effect of photosynthesis in your pond. What? I hear you ask - it's a long story and one detailed in several posts of mine elsewhere on this forum but the crux of the tale is that shade can also help keep your pH levels from varying wildly between day and night. 'Nuff said.
Most guys advocate using a UV as this will certainly help to reduce algae growth and help keep your pond looking clear. They're particularly good at stopping green algae, the stuff that makes your pond look like pea soup if left to its own devices, and especially so if you also have some shade (another good reason for your pergola). For a 2000 gallon pond it won't need be massive - something like a 25w unit would be more than enough I'd say.
Pump size - there's a lot of variables but it's generally considered that you should move at least half the volume of your pond water through your filter each hour. Personally, I pump around double that, i.e. the whole pond volume over an hour as I think that works better for me and I think most guys run at about that on average. I should say though, that in addition to that, I also have about half the pond volume each hour pumped over my little stream feature which augments the main filtration system as it contains my plants and has an alfagrog media base. On 1750 -2000 gallons something between an 8000 and 10000 lph pump will do the job under normal loading.
A pump that size will give you a reasonable flow back to the pond but it's always a good idea to budget for a decent air pump. Again, it's something most guys consider essential. There are times when it's really important to maximise the amount of dissolved oxygen in the pond in order to keep your fish healthy. In hot weather, for example. Also, some medication/anti parasite treatment require maximum oxygen levels during treatment so I'd say you have to budget and plan to use one in your build.
Regarding the extra filtration; that's a personal choice. The anoxic filter system is superb on its own if it's done properly and in accordance with Manky's article and that's easy to do if you're starting from scratch. I augment mine only because the drum I bought as a pre-filter has a small combined fluid bed incorporated in it anyway and also because I can't plant my baskets up currently as the tank they're housed in is in my garage. The beauty of an anoxic filter, apart from the aesthetic benefits, is that it costs literally nothing to run and will last for years. Design it properly and you can add baskets as you increase your fish stock, daily/weekly/monthly maintenance is not required and your water quality will be about as good as it can get.
Hope that helps mate. Cheers,M
