As for bottom-fishing, just follow the main force.
Generally speaking, at the end of a downward trend, if "the trading volume starts to increase significantly", you can follow the position, and the probability of reversal is very high.
Because in a general downward trend, the trading volume is very large, and short positions are closed, so naturally you have to open long positions to close them.
As for how to tell whether the downward trend is at the end, you just look at the length. The length depends on your own feeling. Look at the downward trend marked in the figure below, which is relatively long.
