Over the years? Thousands. But over the last ten or fifteen years, not too much, once you learn some lessons, learn not to be impulsive and impatient, and learn the fundamentals there are far fewer losses.
Also, unlearning bad husbandry from early mentors in the hobby, for example, for the first year I kept a reef tank I followed the advice of this guy who said not to do water changes, but also didn’t implement any other form of nutrient export and didn’t dose anything. He also said testing was unnecessary. I tested my nitrates after a year and they were off the charts, calcium was like 80 (not sure how there was even that much) don’t remember what any of the other params were. Oh, and he also said dechlorinated tap water was better than RO/DI because you want all those minerals. After a year my corals and fish were still alive, but not thriving, and the tank was just ugly. Started asking other people for advice and reading the forums, and realized how off track I/he was. Can’t believe some of the stuff I used to do because it was what I was taught.
