Rain water dilemma

As any city dweller knows using city water can be expensive.  I have a water pond which needs to be refilled to replace evaporation and it can take 500 gallons to refill it, which isn't cheap.  The discount the city gives for water use in a pond isn't worth the phone call and with all the added chemicals city water isn't the best to use even if the chemicals are diluted enough when mixed with the water already in the pond as to not be deadly.

I added lots of salt a few weeks ago to kill of the algae and it did a good job for a fraction of the coast of chemicals but salt in high concentrations also depletes the oxygen level in the pond and sometimes the fish struggle to breath, especially at night so I need to dilute the pond water to lower the salt level. 

I have two rain barrels set up, one for my garden and one for my pond.  When I don't need the water for my garden I'll drain that water into the pond.  The bigger barrel has two overflow valves which I have rigged to drain into the pond.  Any significant rainfall will quickly fill the barrels so in doesn't concern me to empty them if I need to but after the barrels are full, any rain above a sprinkle will bring water down the drainpipes faster than the overflow valves can deplete the incoming water and much is wasted.  The small barrel doesn't have  overflow valves nor does it need one being as it is on the higher end of the house and doesn't fill up as quickly. 

I do have the drain plugs which on the big barrel is already affixed with a drain hose that goes into the pond but because of the distance I don't keep a drain hose on the small barrel.  I usually dip the water from it with a bucket to water my plants with instead of with a hose. 

I just hate to see water wasted and if the drain plug was opened on the big barrel less water would spill onto the ground and more go into the pond.  But who wants to run out during a downpour to open a valve?  Sometimes I do but occasionally when I do I don't get the valve closed in time to keep it from emptying the barrel once it quits raining.  Sometimes I'm asleep and others times I have other places to be. 

Rain is so unperdictable here that even if the forecast calls for a 100% chance of rain it isn't always advisable to open the drain plug in expectation of rain refilling the barrel.   Things often just don't work out that way.  I'm sure if I had the money I could rig a system in which the drainplug could be opened automactically or a control valve could be installed in the house but I'm a poor boy so that just isn't going to happen.