The LAST big upgrade

[Delphinus]

I had to remove a butterfly fish from my big tank recently. I knew from the time I put him in that I would have trouble when it was time to get him out, but I needed him to serve a purpose (which he did well) so I tried to ride it out as long as I could, but it became time this fall. I had to use the clear birdnetting that you sometimes see used for tank lids. I had to cover the rocks and close off all the gaps and caves. I ended up using all of it and it still took me 1.5 hours but eventually he was cornered and had no further escape route.

Or you could try a fish trap. The way I have my tank in my wall without front access means I can't use a trap to remove fish from my big tank. But otherwise those are said to work well..