You need to see how the TDS fluctuates between water changes. It will give you your answer and help maintain the TDS stable via enough water changes. The crushed coral may help some stabilise things, especially if your well water has low alkalinity.
You may not consider your tank overstocked, but you've got large fish that are heavy producers of waste and its the equivalent of stocking a small tank with a bunch of livebearers. My 240G pond has 30-40 fish, the smallest are 6 harlequin rasboras, next biggest 9 denison barbs and the largest two of the clown loaches, one around 8inch, the other 9inch. I consider my tank fully stocked right now.....For 40G more water you've got arowana, oscars, etc..You just need to work harder to maintain such a tank..Its a choice we make...