Algae / tank help

[Marinebeginner]

My tank is just over 4 months old so I'm still just a baby and need some help!
I'm getting quite a lot of green algae spots on glass and on bed of tank (not hair algae) - this is despite a big clean up crew including a sea hare. ( see pics)
I do have my lights on for about 12 hours a day but I think it has started more since I started adding nori for sea hare - which is more likely?
I have been doing 10-20% water changes once a week
Parameters: pH 8.0, nitrites 0, ammonia 0 but nitrates 25 (eek!) - about to do another water change. My nitrates normally sit about 2- 5ppm. Again is this because I am over feeding??
All advice gratefully received - appreciate this is prob simple beginners stuff!

 

[Sniperhank]

Might be from over feeding but could also be from junk built up in the gravel bed you have.

 

[maxb]

You need to bring your nitrates down. How big is your tank? What's your filtration setup?

 

[Marinebeginner]

It's a nano tank , boyu TL 450 so has its own filtration system, I replaced all the plastic balls etc with chunks of live rock on advice of LFS when I started. I've added extra powerhead as boyu original one pretty pathetic.
Nitrates have never been that high before (well since first cycle). Did a 25% water change last night. Will recheck levels this morning.
There is always algae stuck within my gravel these days - another new thing. Should I change to sand substrate? How on earth do I do that without causing mayhem in the tank ?!

 

[Digital Deviant]

On a small tank it's very easy for nitrates to ramp up without regular large (50%) water changes. Is the tank in much natural light?

Changing substrate will be tricky but will help a great deal as all the material in there will break down and raise the levels of nitrite/nitrate.

 

[Sniperhank]

CHanging substrate won't be too bad. You'll need a piece of PVC to stick down into the water to pour it through. If you place it down close to the bottom it will lessen the cloudiness that will come from adding the sand.

 

[maxb]

Def do a large water change. If you keep having issues with nitrates reduce your feeding a bit and maybe check your ro water filters

 

[Marinebeginner]

Have done a water change but nitrates still 10ppm. Cutting down light and amount of feeding. Will have a trip to LFS this weekend to speak to them about changing substrate. Maybe if I do it in stages?
I buy my RO water from LFS. Haven't ever checked it for nitrates but will do now !