We hear fertilizers do not cause algae time a time again. This is true in respects to the fertilizers we intentionally add such as KNO3, Plantex etc. However, we need to remember why the notion that nutrients cause algae began.
Years ago hobbyists kept fish and plants together much like we do now in low tech tanks. The advanced hobbyist even tested water parameters like NO3 and PO4. They noticed that when they had high levels, algae would become a problem. They also noticed that these "bad things" would appear faster when we over fed, didn't clean our filters, failed to remove decaying organic debris etc. Sounds like a no brainer to blame algae on excess nutrients right?
Fast forward to today. We see beautiful algae free tanks with ridiculously high levels of nutrients we intentionally add without algae! That basically proves that these components alone do not cause algae. But why?
Think about the source of the "bad things" in your great grandfather's aquarium. It certainly wasn't dry fertilizers. Instead, the "bad things" were a result of decaying organics.
If we toss a chunk of fish in our tank it decomposes. Sure we see increases in the things we want like NO3 and PO4 but that's not all. There's also a slew of "other things" that decomposing organics add. These "other things" contribute to algae.
In a well established aquarium we have a wealth of beneficial bacteria that feed on these organics. Not only the bacteria we all know of that convert ammonia to nitrate but a real ecosystem consisting of bacteria, fungus and other things we've probably never heard of. That balanced system takes time to establish. In fact, it takes much longer than the time it takes to establish the colony of ammonia munching guys we pay so much attention to.
The microscopic ecosystem can help recycle organics that tend to cause algae. However, when we have a new aquarium we have to pay particular attention to keeping the "other things" very low. The easiest way is to frequently change the water. We can also reduce the organic input leading to the "other things" by reducing the food we feed our fish. Even an established aquarium can become overwhelmed by these "other things". When that happens we see algae.
One last question. I seem to be getting the heaviest amount of algae right where
my returns shoot out into the tank. Why is this?
Let's use an analogy here.
Imagine a city full of people who cannot leave their homes and eat only pizza. Where would we expect to find the largest population of over weight people? Simple. Where there are more delivery guys!
If that city had pizza eating rats where would the rats travel to?
Think of water flow as the pizza delivery guys. Water carries the pizza. So we would expect to see algae in areas of higher flow. The nutrients are constantly being resupplied. In fact, the increased pressure of these areas causes a steeper diffusion gradient. Which basically means it's easier for algae to uptake nutrients.