Sounds like you've got one determined kitty. When my tank was still empty (with an open top), my cat would always jump up there and sleep on the center brace. After I filled it, I expected she'd end up taking a swim... lol. But I added a 322 watt light fixture at the same time, and that stopped her. She's not gone up there yet.
You have two things going on with the cat to deal with... one is simply curiosity and the other is that pouncing instinct. Some reverse psychology might help. Instead of working so hard to chase him away (which usually only makes a cat more determined), encourage the cat instead. Let him know it's okay to watch the fish.. as you did putting a wider ledge where he can watch. Before you go toward the tank to do something there... pick up the cat and take him with you. Hold him close to the tank.. say reassuring things.. make him feel like he's welcome to be there and included as a participant in what you're doing. Really, go all the way with this. If you're working with plants, a fish net, whatever... hold that over to the cat and let him sniff it. (Accidentally dropping a few water drops on him at the same time.) This tells the cat that this tank setup "belongs" to him. This lessens his curiosity factor.. he'll get tired of being held up to see what you're doing.. and he'll rather be put back down.
At the same time, set up a real shelf or some cat furniture near the tank. He's less likely to climb up on top when he can sit or lay comfortably to watch the fish. Putting a circle of crumpled up aluminum foil around the top edge of the tank (temporarily) will also discourage him from getting on top. Make sure you let him feel that foil along his fur, too.. when you install it.
Don't know how well any of this will actually help.. let us know how it goes.
You have two things going on with the cat to deal with... one is simply curiosity and the other is that pouncing instinct. Some reverse psychology might help. Instead of working so hard to chase him away (which usually only makes a cat more determined), encourage the cat instead. Let him know it's okay to watch the fish.. as you did putting a wider ledge where he can watch. Before you go toward the tank to do something there... pick up the cat and take him with you. Hold him close to the tank.. say reassuring things.. make him feel like he's welcome to be there and included as a participant in what you're doing. Really, go all the way with this. If you're working with plants, a fish net, whatever... hold that over to the cat and let him sniff it. (Accidentally dropping a few water drops on him at the same time.) This tells the cat that this tank setup "belongs" to him. This lessens his curiosity factor.. he'll get tired of being held up to see what you're doing.. and he'll rather be put back down.
At the same time, set up a real shelf or some cat furniture near the tank. He's less likely to climb up on top when he can sit or lay comfortably to watch the fish. Putting a circle of crumpled up aluminum foil around the top edge of the tank (temporarily) will also discourage him from getting on top. Make sure you let him feel that foil along his fur, too.. when you install it.
Don't know how well any of this will actually help.. let us know how it goes.
