Very dirty glass

Did you try using a sharp razor? Fill the tank with water and use some sort of sharp edge to scrape the glass. ADA sells this tool and I am sure there are others.

http://www.adana-usa.com/index.php?main_page=afa_product_info&cPath=6_57&products_id=341

It isn't a crust of calcium, it seems like it has seeped into the glass.

I have had tanks and lids like that and have never had luck getting rid of those stains, I have even tried muratic acid which usually works like dynamite. I think when hard water stains on glass are that bad you're just out of luck.

If it is etched into the glass, I don't think you could remove it.

It is not calcium deposits. It is etching of the glass, itself, by the water over a long period of time. Glass is a wonderful product, but it is slightly soluble in water. I have some old tanks where the glass can be seen to be slightly cloudy when dry. When filled with water the glass still looks clear. However, at the tops of these tanks, there is a lot more etching and depositing of silica by the water line, which moves up and down, leaving about an inch and a half of more seriously clouded glass. There are polishing compounds that can be used to polish the glass, but you need an electric drill equipped with a polishing attachment. Progress is agonizingly slow if you try to do it by hand.

Is the grit safe for the fish? And is the etchin on the surface or further down? I can get a random orbital sander for 30$ and some 27000 grit. Will that work?

I had the same problem on my 46 bowfront and lived with it for 4-5 years. Some one on here suggested steel wool so I gave it a shot. It really couldn't hurt right? It took about 3 or 4 scrubbings every week with water changes but it did work. It's not going to scratch your glass and it worked great for me.

If the tank is empty carry it outside and wash it well with Brillo,
that will clean it up. Rinse well, it will be fine.

Hmm, the tank is empty and has been for months. The soap will be ok? Won't the metal scratch the glass? My razorblade has scratched the glass before when I used to scrape GSP off.

Hi Zapins,
I agree with Rupey, try fine steel wool.

Zapins said:

Hmm, the tank is empty and has been for months. The soap will be ok? Won't the metal scratch the glass? My razorblade has scratched the glass before when I used to scrape GSP off.

Click to expand...

Yes the soap will be okay, the brillo will scratch paint, not glass, get you a drinking glass
or wash a window using brillo see if it scratches it. Rinse well, it will be fine and will clean
the glass like new money.

I want to add to this simply for clarification>

When you do clean the glass with Brillo, it will clean it so well
that it will actually reveal to you just how many flaws and
scratches already exist on that said piece of glass....
Be warned

Woah... that is some strong stuff!

I'll have to try that then.