Also you have no way of knowing your dilution rate just by chucking a bit in the tank every few tanfulls gives you no idea of how strong your solution is. If it's floating on the top and not evaporating anywhere then your solution will get stronger over the months. The only way to accurately measure what your doing would be to measure the specific gravity with a hydrometer at the point where the water contacts the glass before evaporation - you'll probably find the reading so small that your using pure water anyway.
Isopropanol by the way is excellent stuff!
Appearance: clear. Flash Point: 12 deg C.
Warning! Flammable liquid. May cause skin irritation. Hygroscopic. May cause central nervous system depression. May cause kidney damage. May form explosive peroxides. May cause reproductive effects based upon animal studies. May cause severe eye irritation and possible injury. Causes digestive and respiratory tract irritation. Target Organs: Kidneys, central nervous system.
Potential Health Effects
Eye:
Produces irritation, characterized by a burning sensation, redness, tearing, inflammation, and possible corneal injury.
Skin:
May cause skin sensitization, an allergic reaction, which becomes evident upon re-exposure to this material. Prolonged and/or repeated contact may cause defatting of the skin and dermatitis. May cause irritation with pain and stinging, especially if the skin is abraded.
Ingestion:
May cause gastrointestinal irritation with nausea, vomiting and diarrhea. May cause kidney damage. May cause central nervous system depression, characterized by excitement, followed by headache, dizziness, drowsiness, and nausea. Advanced stages may cause collapse, unconsciousness, coma and possible death due to respiratory failure.
Inhalation:
Inhalation of high concentrations may cause central nervous system effects characterized by headache, dizziness, unconsciousness and coma. Inhalation of vapor may cause respiratory tract irritation. May cause narcotic effects.
Chronic:
Prolonged or repeated skin contact may cause defatting and dermatitis. May cause allergic skin reaction in some individuals.