How to remove really persistent lime (or silica) scale?

[Red Drum]

Hello, all, new user here.

I did a search on this question, but didn't find anything particularly useful. I have a 30-gallon glass tank that has sat neglected for several years. I'm trying to get it back up and running, so I cleaned it out real well and refilled, and discovered that there is some kind of mineral scale on the glass near the top of the tank. We have really soft (well) water here so I'm not sure how it got there.

Long story short, I ended up taking the tank outdoors, taking everything out of it, turning it on its side and putting concentrated (~35%) hydrochloric acid on the glass to try to take off the scale, and it doesn't seem to TOUCH the stuff! I tried to scrape it off with a razor blade, but this deposit seems to be as hard as glass. I'm thinking it must be some kind of silica deposit (I would expect HCl to eat lime scale in a heartbeat), maybe from the gravel I had in the tank? Short of hydrofluoric acid (which I believe will eat the glass, too), does anyone have any tips for getting this stuff off, or should I just learn to live with it? I'm trying to avoid things like ScotchBrite, since I've seen how they can scratch glass...I even tried a tungsten carbide scraper, but no luck. (I'm reminded of my father's experience working in a bone gelatin factory where they tried to scrape hardened gelatin off of window panes, and it tore pits out of the glass!)

Thanks for reading my post, and TIA for any tips. I'm looking forward to reading and learning from all the more experienced members here.

Best regards from Virginia,
Red

 

[AilyNC]

Vinegar & baking soda?

 

[StevenF]

If it was lime hydrochloric acid removed it. I wonder if the acid you used etched the glass. HCl won't effect siO2 but gas is not 100% SiO2. IN the future vinegar and citric acid will remove lime with less risk of damage to glass.

 

[PheonixKingZ]

Vinegar worked for me for my 20g long tank.

White vinegar mixed with a razor blade.

 

[Red Drum]

Thank you for the replies. I tried vinegar and baking soda before breaking out the HCl ... no go. This one has me stumped. Just for grins, before I put it back in service, I'll try degreasing it with a strong base like NaOH and then try acid again, just in case there's something in the deposit (some kind of oil?) that's shielding it from the acid. I don't get it ... HCl should remove anything short of silica/glass...

 

[thuctoanvietnamese]

Lemon juice worked for me
Are they green ?

 

[mcordelia]

Yes it does sound like you've taken out the big guns. I wonder if it's some kind of an acidic residue or a less acid-reactive dried-on deposit. The fact that it's at the top of the tank makes me think it got there due to evaporation (sorry if I'm stating the obvious), but that would then suggest thinking about what else is in water besides acid-soluble minerals... do you happen to have a picture?

 

[Red Drum]

I allowed the tank to dry out so I could better see what's still on there. It's still there, and on all sides, although the acid did seem to reduce the scale on the front (I had it on its side with acid soaking on it). Here's a link to a pic (I propped up a black roofing shingle inside the tank to give some contrast to the white scale on the glass):

https://i.ibb.co/vq5tSFd/tank.jpg

Unfortunately, this tank was left fallow for several years, and 80% of the water evaporated out of it. One of the reasons I bought a glass tank in the first place was because I figured it was better for getting off algae or lime scale (with a credit card scraper or sponge) than acrylic, which can scratch. Whatever this material is, it doesn't want to come off! I have oven cleaner (NaOH) on it now, and if that doesn't cut it, I guess the tank is shot.

Thank you all again for your replies.

 

[StevenF]

Am I correct in assuming that you wiped down the outside of the aquarium with an acid solution?

 

[Red Drum]

Am I correct in assuming that you wiped down the outside of the aquarium with an acid solution?

Not so much the outside as the inside, but I'm sure some also got on the outside of the glass. The scale (or whatever it is) is on the inside, and you can feel it with your fingernail. (HCl won't hurt glass -- hydrochloric acid is often sold in glass vessels -- although HF, or hydrofluoric acid, will eat glass and is sold in plastic or rubber vessels for that reason.) Whatever the case, the scale was there before I hit it with acid, so the acid isn't what caused what you see in the photo.

Update: It almost seemed like the strong base (sodium hydroxide, in oven cleaner) might be getting some traction on whatever the stuff is, but now I'm not so sure. I do know that when you apply certain chemical coatings to metals (for example, bluing on guns, or phosphating on steels to make them rust-resistant), the normal routine is to degrease the metal with a strong base, like sodium hydroxide, to remove any oils that would shield the metal from the acids that follow, then apply the acids (or other reagents), which then can act on clean metal. I'm hoping maybe there was some kind of hydrophilic compound (oil/grease/wax) in the scale deposit that was shielding it from the HCl, and that I removed it with the NaOH, thereby allowing HCl to remove the scale. I'll put some more acid on the stuff tomorrow and see...if that doesn't work, I guess the tank is done-for, which would be a big disappointment.

 

[mcordelia]

curious to know if your two-step approach worked