Sorry, degrees/drops. Not ppm.
So basically clear cement.
I am going to assume it is 40 German degrees of Hardness, based on 2 factors:
OP asks 'how toxic is it?' and that implies a high level.
Wells are often in limestone or related rock, and this adds GH, KH, and raises the pH.
However, the other answers are exactly right: Always include the units!
That high level might be a problem if the Ca:Mg ratio is very skewed, but it would have to be way off. Plants use Ca and Mg in a ratio of 4 parts Ca: 1 part Mg, but the water does not have to be that ratio. Just so long as it supplies both in a reasonable ratio. When either are extremely out of line the excess can block the uptake of the other.
The high level could be a problem for some of the highly specialized plants that demand low mineral levels (especially low Ca). These plants are not commonly seen in the aquarium plant stores, though.
I do not know if this happens with Ca or Mg, but...
When some minerals are present in excess they block the plant from picking up certain other minerals. Gotta look into this more, and see if it can happen when Ca or Mg are high.
Yes, it can.
Excess Ca can block the uptake of Mg, and B.
Excess Mg can block the uptake of Ca.
If other minerals are in excess they can block the uptake of...
..K..........N, Ca, Mg
..Na........K, Ca, Mg
When the GH is high because of limestone in the aquifer, the KH is almost always really high, too. What is the KH? This would also imply that the pH is very high. (is the pH over 8.0?)This would be more of a problem. At high pH many of the minerals plants need are chemically bound up in a way that the plants cannot get them. Nutrient deficiencies would show up, even if the element is present in the tank.
Wow, thank you, that was highly informative. I started to check the KH and after 20 just tossed up my hands and said "what the heck is this stuff?!" (our city tap is somewhere around 8kh and 12gh I believe) and gave up, because I wasn't looking to figure out the source water stats at the time, I was just ensuring my own dosing of the tank was correct by making sure I hadn't given too much Equilibrium, which I have to use a tiny bit of in my own (city tap) tank. When it got that high I realised there was WAY more going on here than my adding 1/8th tsp Equilibrium to our city tap and it took me some time to find out the water was well water. This also explains the high nitrate contents in the tank, despite high filtration and sparse feeding, as the well water tests out at ~9ppm nitrate. When you top off regularly and have no plants, this number climbs. As a result, I spent a LOT of time last year doing 50% water changes on this tank, in an attempt to keep the Nitrate from hitting toxic levels. I always assumed the fish were over fed, but now I see the truth.
I'll check the pH and the Kh and get those numbers posted as soon as I can. This helps explain a LOT of strange things that happened in this tank over the last year. Always, always, always check your water source before assuming you know what it has in it I suppose.