High nitrates :(

[dynamite]

Hi, last time I tested my Nitrates which was a long time ago, I used to keep them at 0... (even tho I had some hair algae)
A few months ago I started to get hair algae and cyanide no matter what I did, but never checked my Nitrates or phosphates.
I was tired of my tank looking so ugly So last friday I went to my LFS to buy some biopellets, I showed the guy a few pictures of my tank but he said the problem was that I didn't have enough LR and that LR is suppose to be my natural filtration, he suggest me to start by adding 16lbs of LR.. and then to get 16 more.

I came home and changed the 10% of my water and now my water looks crystal clear like it never did before So today I decided to check my parameters and I was surprised to see my Nitrates were between 20 and 40 (It's not a dark red and it's not a dark orange It's in between) and my phosphates were at 0.03 tested with my Hanna Phosphate checker.

I only have 3 fish. The hair algae seems to be dying but I still have some cyanide in my old LR and base rock... I'm I having high nitrates because I just introduced LR to my tank? Do I need more LR? Thanks again!

Here's a picture of my tank, if you guys need any extra info just let me know, thanks in advance!