I have electric running to my barn that has been in place for 5 years with no issues. This year when we plugged in the tank heaters the horses are getting shocked. I tried disconnecting all other circuits and tested with just that one circuit and it is showing 3.5 volts in the water. It's there even if the breaker is turned off and the heater is in the tank. Disconnected the black wire in the garage panel and with the main breaker in the garage panel turned off, I get the 3.5 volts when testing between the neutral bar and the black wire. Turned off the circuit in the main panel in the house going to the garage and again, still getting voltage coming through. Any suggestions??

Have you replaced any copper plumbing with PVC in the barn recently?

If you did, then bond a copper jumper wire across the PVC to reconnect the grounding system.

Do you have an electric fence nearby? Possibly getting some shocks from the condenser through a ground in the water? The water tank should ground itself if it is metal.

Here is some light reading on the issue:

http://www.pge.com/includes/docs/pdfs/about/news/outagestatus/powerquality/power_quality_bulletin-issue_no.2_stray_volt.pdf

You need to look for the problem at the source, I will assume the horse barn is fed by home.

Without any further information, does sound like a grounding problem.

3.5v between the disconnected neutral wire and the neutral bar?
Have you tried measuring current between these?
Getting voltage between the water and the heater without the breaker tripping points to an issue in the breaker area.

It could be as simple as a faulty breaker (they aren't always perfect). It could also be a wiring mistake, somebody got the live and neutral switched somewhere in the circuit (probably in the breaker box). This would take the breaker out of the equation altogether.
You wouldn't ordinarily notice this as pumps and heaters aren't bothered about polarity.
If you get current between the wire and the neutral bar, one of these is your problem.
If you don't get current between them, the problem is isolated (electrically). In that case the only culprit has to be the heaters.
Bear in mind that even after they are unplugged, they can exhibit a voltage for a time. The rule of thumb is 15 seconds, but I've seen heating coils in some circumstances hold voltage for up to five minutes.

In the mean time, you could use some aluminium foil, folded into a strip, one end in the water, the other with a nail through it into the ground to stop your horses getting shocked.