Right now I'm using a cheap 240 page AmazonBasics Classic Notebook, but I'm already like halfway through it and I have had it for less than two months, and the paper quality isn't all that great. When I fill it up, I might get a 480 page Seven Seas writer notebook, but I don't like how it doesn't have a bookmark, or a hard cover. I also might get a Leuchtturm1917 notebook, but it doesn't have that many more pages than the AmazonBasics one.
I started one while I'm doing a fish in cycle in the new tank and find myself writing all sorts of things in it about the others. Coloured good quality writing pens make life easier too especially when you need to note or highlight something.