We just had this discussion in another thread. Clay that has been heated to super-high temperatures doesn't present any danger of reacting underwater. Just don't add a layer of soil underneath the tiles (we went back and forth for half a page over that one issue in the other thread.)

 

I'd skip it all together. You're just introducing another thing in your tank that may cause problems down the road. You'd have to tear down your tank to remove these if they become an issue.

 

Why? That is not a danger. It is benign if just put in right. What is the purpose of putting them in?

 

To add on to what @maico996 suggests - another thing to consider is that a 1-2" slope will likely not be noticeable once substrate is over top and plants begin to grow.

 

Ok I will skip the tiles. Thanks for the advice.

 

You can put gravel in filter mesh bags or nylon. Use that to add depth.

 

If you put the tiles on top of the glass and then add substrate, I don't see a problem.

 

I'm a bit late to this thread, but I used 1-2" of ceramic tiles in the back of my 20g tank to create a slope. It has worked out great so far, no issues about 7 months in.

 

No issues is always good to hear, but I wouldn't have expected any. [Tiles are inert (i.e., won't decompose like wood or soil organic matter in soil).]
Feel free to show us a picture of the tank with it's tile-constructed slope.

 

I realize this may be an old thread, but I have a question. I have a large rock I intend to use in my next aquarium build that weighs about 35 lbs. It‘s a magnificent Pa Hai stone, and I’m concerned about placing it directly on the glass, and also do not want to create a problem with anaerobic activity beneath it. How would be be to support it with Bio Balls, and then add soil around the rock? What do you think? Ideas and comments please… and thanks in advance!

 

I think the bio balls are a great option for building up substrate. And it seems reasonable to use them to protect the glass against scratches. But 35 pounds of pressure against glass? That's above my pay grade.

 

Thanks for sharing your thoughts. Yes, I get that about the 35#. I may have to rethink that. It causes me concern too.

 

I don't think the weight of the stone is a problem. What shape is the bottom of the stone? If it is flat so that the weight is spread when resting on the glass it should be safe. If the the stone rests on only two or three points, then it would be good idea to place something flat and rigid under it to spread the weight. A ceramic tile would be perfect, or even something like a rigid plastic cutting board.

 

Thanks Michael. It is more rounded, but irregular, and will likely have no more than 3-4 pressure points. It would probably be a good idea to get a 12”x12” tile to place under it to distribute the weight more evenly over the glass. (Thanks Diana too!)