check coolant level keeps coming on

Ach! My bad. I missed where you just replaced the tank (which would have automatically come with a new sensor). Sorry. Okay. Two possibilities I see: (1) Contamination of new tank, and (2) an electrical problem. (1) Did you flush the entire cooling system (engine block, radiator, etc.)? Since you just replaced the coolant tank (which would have come with a new sensor) it is unlikely to be the sensor. However, it is possible that the gunk circulating in the cooling system just found it's way into your new tank and jammed up the sensor float. The next step would to flush the new tank and ensure the float is both moving and activating the sensor properly. Clamp off the hose to the radiator and disconnect it from the coolant tank (capture the escaping coolant as best you can). Then remove the tank and flush it with clean water. Make sure the coolant level sensor float is moving freely (and more flushing while the float is moving about). Connect an ohmmeter* to the sensor and made sure the float is activating the sensor as it moves up and down. Reinstall the tank, reconnect the hose from the radiator, take the clamp off, refill the tank to the proper level and see if the problem is gone. If not ... (2) Since we tested the switch in (1) it is possible that all the manipulating of the harness has damaged the plug / harness in some way. Now you'll need schematics (good luck) and some troubleshooting skills. * CAUTION: Do NOT use a regular test tool (battery, bulb and sharp prod) as this sensor uses a reed switch which will likely get damaged using simple lamp-based test tools. An ohmmeter set to check continuity (ohms) is the safest way to test whether or not the sensor is opening / closing as the float (magnet inside) passes over it.