The API master test kit for freshwater is around 20-40 dollars. It usually hovers around 30 something in the stores by me.
If you can get a picture of bluefish that would be really helpful. The yellow spots you said he developed doesn't sound really fun. Without a picture on our end, look up velvet disease in bettas, try to compare pictures between the disease and your fish maybe? Loosing chunks of fin could be a few things. A picture is worth 1000 words though in many cases. Some bettas have irridescent coloring though which could look like velvet if you haven't experienced it before.
Before you panic though and start treating for something your fish may or may not have really try to get a picture if you can. (and filling out the information on the sticky post will expedite the help you will receive. It just puts the information in a format people are used to reading and can see problem areas faster.) Many people have pasted the sticky information in their thread then changed the text color to type in the information right into the form.
Some fish are more prone to being sick. It sounds like poor bluefish might just be more delicate then redfish but everyone here will try to help get things sorted out.
Did the drops make the water blue and is the container shaped like a little fish?