I am currently using an ionizing water filter system (Bel Ozono sold in South America) with two filters to treat water from a cistern. My concern in addition to giardia is decomposing animal matter (insects, lizards, snakes that fall in and drown). I wonder if there is a more effective undersink system. Have suffered no stomach ills so far using the current system but would like to install the all out best.

I am currently using an ionizing water filter system (Bel Ozono sold in South America) with two filters to treat water from a cistern. My concern in addition to giardia is decomposing animal matter (insects, lizards, snakes that fall in and drown). I wonder if there is a more effective undersink system. Have suffered no stomach ills so far using the current system but would like to install the all out best.

Is this "cistern water" used for drinking as well as washing, bathing etc?

Yes it´s the only water source. Actually water arrives at a concrete holding tank via gravity from a spring and is then pumped to another tank on the mountainside above the house so the house is gravity fed also. The Bell Ozono system is installed at the kitchen sink and supplies drinking water only.