The bleach water combo is great, in fact my farrier recommends it over any otc thrush fixes. The only way it hurts the horse is if you get it on their skin... which is pretty easy to avoid by holding the hoof at an angle and spraying a light stream downwards into the hoof. For the bleach to water ratio: Get a spray bottle, fill just about 1/4 with bleach, then fill and top with water. It should smell like weak bleach.

If you're still not comfortable using the bleach, switch to straight iodine instead of betadine. How are you applying it? If you're just squirting it in, look for a dull ended syringe and use it to poke and apply deeper.

I used to think that thrush was a terrible, terrible thing. Now I've realized that it's just a fact of life. Some people get athlete's foot in the summer, some horses get thrushy in the winter. The best you can do is try to manage it and not let your horse get ouchy and tender.

Hope this helped...
Amy