step ladder......
You really want to get some of those long-handled Pik n Grip type tools. Also some long handled scrapers. You need access from both the front and back of the display tank. But most important of all is that you need a second pair of eyes, as the deeper the tank is the more impossible it becomes to see what you're doing down at the bottom of the tank.
Dave.M
Yes those tools for the 'old' people that let you reach something at a distance. (I have two of them)
A step ladder that you are comfortable standing on.
A table that is tall so it can be within arms reach of you picking or putting something on it.
A 1x8 cut to slightly larger than your tank so you can put it, running front to back, on top of the tank so you can have a very small workspace right there. Good for moving rock where you have to cross some braces and your arms have a hard enough time fitting. Not for sitting on, just large enough to put a rock slide it down, or move your own ladder, and then pick it back up and place it.
The 1x8 idea is great!
I half wanted to build something so I could lay down and look down into the tank to work, like suspend a massage table, lol.
What is a good grabber arm? I have one but it sucks for anything more than picking up the tongs if I drop them in.
Your tank is actually 280 gals. All good tool suggestions above and available at most online stores or LFS