That would worry me ... This might be just some accidental scratches, or they might be pressure cracks due to stress from the acrylic not thick enough for the water depth. <A 150 gal
is maybe 24-30" deep, & should have 3/4" thick material.> Another possibility is that the acrylic is extruded & not cell-casted. The extruded stuff get fine checks in use & eventually turns hazy (but still structurally sound), but I don't think the checks should be deep ones like yours.
Certainly reinforcing the bottom is a good idea. If you solvent weld a strip along the bottom (I would do all 4 sides .... if it has pressure crack on one side, it will prob happen at the other sides too.), you would in effect increase the thickness all along the bottom edge, and create a kind of a rim to reinforce the tank. This maybe overkill, esp. if the area was cracked because of a blow, but you prob. sleep better at night doing it.