I had a very similar problem with double 0s, it was causing massive chrysophytes growth and I though they were to blame with the low nutrients. Scrubbing the rocks every weekend became a royal pain and I'm naturally a lazy reefer. Even the 10 hermits and 20 snails couldn't keep up. Come to find out my tank was too clean so changed my filter socks out less often and I turned off the skimmer. Huge improvement in just a couple of weeks with algae growth and the corals started to grow too. I plan to turn the skimmer back on but, I need more bio load first. My "bio load" comment is probably true, but I've been using it on the wife so she will allow me to buy more fish.
When I first got into the hobby, I had bad algae issues like everybody else. I ran into a YouTube video where the guy was saying that algae can’t survive with low N&P number. The main number mentioned in the video was get phosphate below 0.03ppm “and algae can’t survive.”
Let me tell you from experience that many algae species can absolutely live and thrive with phosphate at 0ppm and nitrate at 0ppm. You can even cut the lights for weeks and it will still survive.
I realized later that I was a victim of misinformation.
That fueled my passion for nutrient levels and nutrition in our home aquaria.
So the question that everybody always asks is how to balance nutrients and what are the ideal levels. The Redfield ration is often mentioned, but is a little extreme for me. I find that PO4 of around 0.03 to 0.1ppm and up as far as 0.15ppm is great. After 0.15 (depending on lighting and nutrients) algae will start to grow too heavy sometimes. I like to try and target PO4 around 0.05 to 0.1 ppm. I like nitrate at 3-15ppm. I have good luck at around 5-8ppm, but try to keep the ranges above.
The way I will setup my new system is really easy. The rock is currently 9 months cured and established with live sand and colonized Siporax. I’ll start the system and let the new tank, pipes, overflow, sump, buildup a nice biofilm and stabilize. Then add my fish load and start testing to see where the numbers are falling. If the numbers are higher than normal I add more Siporax. Later,
if the numbers dip too low I start to remove Siporax until I find the targeted balance I’m looking for.
A new tank will typically see PO4 levels around 0.5-0.8 and sometimes over 1ppm depending on your husbandry skills, rock, feeding, water changes, etc. Mine get to about 0.8 at the highest. A lot of the time they will stay there for about 6 months and then completely drop off. I think this new tank will drop off much faster because it’s basically an insta-tank. If the numbers drop too much you have to act. A lot of newbie reefers never act and in fact keep skimming 24/7, bring GFO and Carbon online, start Bio pellets, etc. This can get them into big trouble.
I think the key here is to know why you are doing what you’re doing. Do you really need the Carbon and GFO? You gotta balance the system. If you don’t, corals decline in color and weird algae takes over which is extremely hard to eradicate. Sometimes at that point is almost easier to start over.
