The Basics - Ponds each day tend to naturally collect nutrients and continually recycle them. The rate at which ponds collect nutrients is dependent on where and how the pond is built.
Nutrients grow plants as nature's way of doing things. Keeping a pond clean of weeds/algae is very similar to keeping the same size area of bare dirt as weed free. A very difficult on going project.
There are various tools to keep the "weeds" to a minimum in both ponds and bare dirt areas.
Your 'newish' pond seems to have quite a bit of filamentous algae and very likely some underwater growth. Chemicals can be a quick way to TEMPORARILY reduce the weeds. Nature will always want to have weeds-plants return to 'feed on' and utilize the nutrients. I tend to minimally use chemical algae control because the pond ecosystem has to "process" all the harsh chemicals that are applied to the pond. Most chemicals not only act on the intended target problem but also affect other sensitive living things in the pond biology. ALSO what happens to those chemicals that are continually dumped into a pond with basically no 'flusher' drain??. Chemical fate is an important part of our environment.
I currently like the algae & small weed natural control of using tilapia. They will actively eat a lot of algae and more delicate weeds. Each fish eats around twice its weight a day in algae. Thus the more algae you have the more tilapia one needs. Suggested starting rate is around 30 lbs per acre. Tilapia will convert the weeds to fish biomass plus some manure. Ideally you want the manure to feed a phytoplankton (micro-plant) bloom instead of more visible weeds. As noted a few grass carp (GC) will help eat the submerged weeds. Larger rooted shoreline and submerged plants directly compete with most types of filamentous and micro-algae.
See this link for an example of good naturally balanced pond (RAH Indiana) with minimal algae control problems. Plants compete against plants for nutrients and growing space. NOTE - It is very, very hard to get natural plants established in a pond that has grass carp(GC). Consider that potential problem before stocking GC. http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=39487&Number=514344#Post514344
Look through this PondBoss Q&A archive link to algae types and various control methods.http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92633#Post92633