I installed a 40 gallon electric heater today and now there is very little pressure. Is there an adjustment on the tank? What have missed?

I installed a 40 gallon electric heater today and now there is very little pressure. Is there an adjustment on the tank? What have missed?

The water valve you turned off when you put the tank in? As in you didn't open it back up all the way?

Is low pressure all over the house or only in shower ? Back to you. Milo