Tank_Auth uses native CI captcha, that not perfect. I agree that sometimes it's very hard to recognize what's written on it. Moreover, it may show '0' (a letter) or '0' (zero) and there's no way to distinguish them.
If you met this problem, I would recommend you to play with captcha settings, and firstly to change the font. It's $config['captcha_fonts_path'] parameter in config-file. There are 5 different fonts available, from 1.ttf to 5.ttf. Try 2.ttf, maybe it will be more convenient to you.
The other way to solve the problem is to use reCaptcha. I use it in my project, and it works fine.
