I dislike the idea of abusing your animals (Shoving them off a table, throwing them, hitting them, it only fuels their distrust and hatred for you) so you won't hear any of that from me. How many Gallons is your Aquarium? Is it one of those fancy coffee table Aquariums? 1 Gallon is around 10 Pounds which is the downside if its a big Aquarium and you need to move it.
What I did with mine is pick him up and put him in another room across the house, or put a sheet over him (Worked with my bird-crazy cats and my beloved bird who flew all over the house, was not my idea to get Cats, I'm more of a dog person) which will cause him to lose interest, or you could cover the Aquarium with a sheet (It'll be like a extended night for the fish if you don't have the light on). Of course it depends how interested your cat is with your fish, some cats just will not stop until its gone. I used Cardboard with mine (Flat Cardboard pieces, cats instinct is to ambush their prey, and you can't take that away, which is why they hop in boxes and bags), but then he had to jump onto something to get at my fish. I also put Cardboard boxes all around my Aquarium so they'll forget what their doing and hop in a box instead of pester your fish.
Every time your cat is trying to get at your Aquarium get him off of it as soon as possible. If you cat knows you don't like what he's doing (And he knows your his source of food, or "Master") he will eventually stop. My cats learn very slowly (Which is another of the countless reason I'm a Dog person) so you will have to keep doing it for however long it takes. Of course some natural cat repellent (Like some mentioned in other posts) always helps, and some persistence to keep him away from your fish.
Cats are both Nocturnal and Diurnal so if your Aquarium is small enough you can cover it with a sheet at night when your not awake to stop him.
Long time cat lover.. I've had all kinds of personalities. One kitty did not care about any treatments I've tried, all of them positive. He was very independent and very passionate about his undesired habit. I purchased a unit similar to this
and positioned it where he would go. I tried various sounds from me just saying "NO!" to alarms and annoying buzzes. I decided to try something a little more natural, a short snake hiss. It worked every time. Not really my normal approach, but honestly it was only a few tries before he would just stand 5 feet away and stare at the newly taboo'd area. I got the idea when out in the field and watching him discover the snakes and lizards out there. He really just chose to avoid the area and even figured out where the motion area was. Downside, when we walked by occasionally it scared me and my guests since some of those snakes come inside. It was always a funny story after g-luck!
(edited for those who don't normally have snakes on their property)
If he's bothering your tank he is obviously bored, why not play with him whenever he starts that behaviour? My Siamese kitten needs tons of play time and if she doesn't get it she starts displaying similar behaviours. Could you cover the tank with a board or something similar? There's really no need for negative reinforcement, it only makes your cat associate the discomfort with you, not the behaviour and this can even lead to fear of a particular room which can create a whole host of other behavioural problems. You could try to discourage him with double sided duct tape or citrus as suggested above, these are better options than spanking your cat or shoving them to the ground because it won't physically harm the cat or cause him to associate the negativity with you.
It could also be that he likes to sit up there because he wants a vantage point, do you have any cat trees or other places where he can sit up high in that area? I really think that this is a lack of stimulation though, if you don't give cats something to do, a lot of the time they will find something to do for themselves, and it might not necessarily be something you want them to be doing!
Hello! Owner of 5 fish-loving furbabies here. It sounds like the cat is what needs to be worked on, not the tank. If he's determined, nothing is going to keep him out of your tank unless you learn to work with him. I'd like to point out that Jackson Galaxy is a cat expert and he actually has a video on youtube about keeping cats off of counters, tables, and pretty much everything you don't want your cat on! On a side note, I would never use negative reinforcement with an animal. Spraying with water, spanking, and any other sort of punishment simply does not work with them. Please never do this! Besides making you both frustrated momentarily, it gives him attention (even if negative) and he will continue to do it if you indeed give him this sort of attention! Please be very patient with him as periods of transition are always very hard for any animal. Positive reinforcement is definitely the way to go. You can still say "no" as the cat will understand, but you must make clear what behaviour you want out of him. Calling him down to you in a firm yet calm voice and rewarding him with a sweet treat he loves when he obeys will definitely start helping him get the concept! I have a cat just as mischievous as yours and I completely understand the feeling! If it does really help you feel better, you can move the fish into a room that the kitty isn't allowed into, but please still be sure to keep all these tips in mind as you move forward with your cat. I really hope this helps!
Thank all of you for the replies... I don't believe in abusing my animals and I promise he knows he is very loved (he's currently sitting next to me purring and "making bread" on my leg) I literally swatted him and told him very sternly no... and I has apparently worked, he hasn't offered to touch my tank. It is a 5 gallon tank so it isn't that big and can be moved easily, I just don't have an extra spot to put it. He isn't bored most the time because he is an inside outside cat and chases birds and lizards outside all day, I believe he is just fascinated with the water and he starts off by drinking and then switched to play and it gets kinda destructive. All of my animals are allowed anywhere they want in my house (except on my counters and in one bathroom die to a stray cat that decided to have her kittens in there). I just got in my new lid that covers the entire tank and has the cut out for the filter which I left the flap on to hopefully deter him from getting the filter out and my puppy shredding it with charcoal EVERYWHERE!!! I'm hoping that he will continue to stay away from it