The individual electrolytes don’t add up to 280 ppm. I wonder if the test already accounted for other electrolytes such as dissolved metals, and non-electrolytes, such as suspended particles? What kind of TDS meter are you using?
A TDS of 280 ppm would be considered hard water. If we only look at Ca & Mg for hardness, you have 50 ppm (40 ppm Ca + 10 ppm Mg – by arithmetic of fractions with a common denominator) , which is less than 4 German degree, and that would be very soft water. It can be misleading if a test kit would test hardness by looking at only Ca & Mg when there is a fair share of other electrolytes and particles in the water. If Ca & Mg are the major electrolytes as compared to the rest of other electrolytes, the test may have validity within some range. I agree with you that serious aquarists would consider TDS a better indication for water hardness, especially in the case of your water TDS electrolyte breakdown.
That is an interesting case you’ve brought up. I’d like to see other comments on this issue.
As for high TDS affecting plants, a heavy fertilizer runoff into rivers and ponds can kill aquatic plants and fish near the vicinity, and cause algae bloom. http://www.waterfiltersonline.com/tds-sources.asp http://theaquariumwiki.com/TDS