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are borates advisable both for when TA should be relatively low (swg, bleach) and also when relatively high (dichlor or trichlor), or is there any situation where they should be avoided?
There are no downsides to using borates unless you have a dog drinking lots of water -- more of a problem for pools than spas (obviously). However, borates are most useful when you have a rising pH problem so aren't typically used when Trichlor is used. They are more useful when hypochlorite sources of chorine are used.