How to clean Buildup under spa

[CarrolltonTXPool]

See pictures. Have large buildups under the hottub Rick overflow and along the grout lines on tiles in a few spots near the hot tub. I think I remember MA/water mix in a cup with some sort or brush to clean it.

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[CarrolltonTXPool]

Found it! it was 5-1 or 3-1 water to acid ratio. Worked pretty well on the big areas after applying for a while and scrubbing.