Cactus Help

A few months ago I inherited a beautiful 30 year old cactus. I was given instructions to water it thoroughly once a month which I have been doing. The cactus lives by a large window so gets plenty of natural light. It had a few brown spots on it when I got it but I feel like it is just not looking very happy or healthy. Hoping someone has some advice for me! It also was a bit dusty and I've tried to clean it with a sponge and water but hoping someone maybe has a better cleaning solution for me.

Thank you!
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    Paris Lalicata Hi Sylvia! It looks like the cactus may have developed a fungal/bacterial infection could've developed. I would recommend keeping the plant in good conditions and increase the light to give it the energy it needs to fight the infection and water accordingly. Therefor, give it plenty of bright indirect light to full sun and allow the soil to dry out completely. You'll also want to use any general plant fungicide and follow the label provided for what I think is a 10 day period to further treat the plant and see if it can be saved if this is the case! As far as cleaning the plant, the best solution I think is just diluting a teaspoon or two of white distilled vinegar in the water. This helps rid of any dust, dirt, salt deposits, and makes them super shiny!
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      Courtney Hi Paris, do you have any recommendations for a fungicide that is non-toxic? I have a cactus dealing with the same issue and having trouble finding something non-toxic that isn't super expensive. Thanks!