im confused. if the legs are firmly grounded how can it rock back and forth ? when the tank tilts backwards get someone to hold it in that position, get down on your knees and try to slip paper under a leg to see if its firmly grounded. If the can slide under the legs are not grounded. Simply wined the legs till it comes firmly floored. If the legs are all firm you may have a soft floor bored that buckles when pressure is applied. to test this Move the tank and stand and all sections of the floor and test for movement