what happens?

[fish buyer]

I'd say I agree with most everyone here. Yes the black substrate will change the FH's color, generally making them much darker, and while it works for some it doesn't for most. I would recommend uploading a pic of your FH so we can collaborate with each other and help ya pic out the best color to help boost your FH's color.

As for the spirulina I would just feed it to the FH rather then putting it directly into the water. If it's just free floating in the water collum with nothing there to eat it, it will eventually rot and become toxic. I would just recommend using either spirulina flakes or some sort of pellet that has a decent percentage of spirulina.

 

[The Masked Shadow]

The little betta in the sump

 

[Ruturaj]

I wouldn't add black sand. He already has black background.

Dead spirulina would just degrade in water and might bring overall water quality down. Spirulina is not a special ingredient for color, all algae are, spirulina became popular I assume due to easier availability. Feeding a mix of algae would be better IMO, just go for NLS algaemax.

 

[Magnus_Bane]

View attachment 1455580

Personally I would just stay with the more natural colored stones ya got in there. It's already pretty well balanced with the background. I would however add a clay pot or something similar for the FH to hide in if it feels the need. Without somewhere to run and hide they can become very flighty if scared which can lead to stress related illnesses. It don't have to be much to do the job. Clay pot, rocks, fake plants, a vase, almost anything works as long as it can break it's line of sight from ya.