Bob, start by throttling the ball valve on the boiler outlet 1/2 way (45*angle). Do the same with the supply and return valves on that water tank.(personally, I would have installed the coil in the boiler and put in a cheap electric water heater for the summer). You're trying to heat a tremendous amount of water. Your boiler is capable of it, but, it has to be done under the right conditions and with increased fuel usage.
You can buy a 3 foot lenght of 5/16" or 3/8" steel round stock at Lowes, Home Depot or the Hardware store and heat the end and bend a little hook and a handle at the othe end, with a torch (acetaline) or if you don't have a torch, you have a built in forge in the firebox.
Clean fire and low flow. If this works, ultimately you'll have to make the necessary changes, like installling a low flow pump, or a bypass-return line (to circulate some boiler outlet water back into the boiler).