Awesome man.
Yeah, the light yellow is the by product of organics decomposing and I am told it is Nature's way of protecting the fish from the sun's UV rays.
Yeah, with just a few fish, most people don't need a man made biological filter. Nature will actually create a "biological filter" that consists of various bacteria in biofilm forming inside of piping and on the surface area inside the pond and algae acts as a biofilter.
Now, if you want to push the limits of Mother Nature by overcrowding the pond, then you might want to think about making a biological filter. For a small pond, the most simple one is a trickle tower (google "pond trickle tower"). Trickle tower is where you simply just fill up a pot with lava rock, be sure the pot has plenty of holes on the bottom, and have a small fountain submersible pump pushing water up the center, or however you want to do it, to then let the water trickle try to evenly trickle over the lava rock, and then let the water gravity flow from the bottom of the pot back to the pond. A trickle tower simply adds to the surface for where more biofilm can form. You could also look into making
just a couple of "biocenosus baskets" to be put into your pond; these baskets is the primary mechanism of the
anoxic filtration system ((the Pond Trade article in this hyperlink explains it the best on how to make the baskets)). With a 300 gallon pond, ya really don't need anything big.
Going to get a little technical. If you do choose to overcrowd your pond, then be sure to do a carbonate hardness (KH) test and pay attention to your pH. As your good bacteria reduce Ammonia and other stuff and algae becomes bigger, they produce various acids that could change your pH. Algae could become more of a problem when the water gets overcrowded. When acids are introduced, then the KH mineral chemically reacts with the acid to neutralize the acid so that your pH does not change too much. Eventually, with overcrowded ponds, the KH will drop and you will have to replenish it. If KH drops too low, then bad things can possibly happen to your pH. There is much more to it, but this is enough I think to give you the idea of what happens if you want ot overcrowd your pond and even still have a proper biofilter. It's not a big deal, it just increases maintenance.
Something to think about if you want to add more fish. There are limits to everything so you won't really know until you add that "one fish or fish grow so big" then things start changing in the water chemistry. Also, fish make babies so you can look into getting some dragonfly nymphs that will eat baby fish; not your only option, just all I can think of at the moment.
