I think its a good point that TDS can consist of anything...one person's TDS of 20 if it is all phosphate is a bigger deal than someone else who has a 40 TDS and it is all calcium. But in the interest of safe advice, we have to assume the worst case.

TDS is just a number...and we can say 0 TDS will always be good whereas higher levels are going to be hit or miss.

@binford...you are on DWSD (detroit) water. The Trop is also on DWSD water...so it is no surprise to me that you can get by with 40 TDS. The Trop for example uses straight tap.