need help

[milesj250]

looking at the waterline in my 220g tank (8x2x2) and have notice the waterline changing(left to right). about 3 weeks ago the right side was 6 mm lower(measuring from bottom of trim down). i looked today and the left is 5mm lower than the right end (measuring from bottom of trim down). the tank has been running for about a year. the tank is on two stands, both are level (each one) and together they are level. the tank lower trim on the left side is about 2 mm high than center, the right side is is level and center is right side high. the top trim on the (bottom side of the trim) left side is about 1.5 mm lower than center, the right side is 1.5 mm hight than center and the center is right end high. the waterline is level right, left and center.by sides i mean center to end (4 foot sections), center center out. level used is a 3 foot. looking for some help.