Introducing mussels to a freshwater tank

Spark.

TheSnailsAreFightingBack. Inc
Hi, I am planning on getting probably one mussel soon (Maybe two small ones, Not the biggest tank). I have a few questions as I have done a little research and I'm very new at all this.
I'm not one hundred percent sure about how green water works. I've read that you use lettuce or something like it, put it in some water and leave it out in the sun until..? It turns green? Algae starts growing on the side of the bucket? And to put it in the tank? Should you add it in during a 20% water change or something?
Can i use spirulina powder as a substitute? How does the powder work and is it the same as the spirulina powder sold in health food shops? Would green water be more beneficial for my mussel? And what about the fish and snails? I don't want to end up hurting them by trying to introduce algae.
Also, I read that the mussles need water to be flowing for them to be able to eat properly. I have a filter and a pump and you can see a current that goes through my tank, but it pumps the air at the top of the tank and I was wondering should I look into buying a water jet or something that's situated at the bottom of the tank instead?
And as for the gravel in the bottom of my tank. Most of it is quite fine but I was thinking that maybe i should look into getting some sand for it. Or would they rather it be muddy? I don't know if that would work.
You help would be much appreciated.
 
It's not the algae on the glass/side that they eat. It is the suspended algae (green water) that they eat.
 
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