Help! My rocks are floating!

We have a 150g tank that is visible on three sides so we wanted large pieces of lava rock in the center to act as a background. In CA we had no issue in finding large red lava chunks, in WI it's just not possible to find. But we did find volcanic rock called Featherlight. Unfortunately if there's an air pocket then the 20x18x8 rocks float.

My dh drilled 6" deep holes in the bottom of the rocks but they still are floating.

Question:
In the case we can't get the air out, what kind of weights and ties can we use? Dh is worried about them deteriorating or effecting the water quality.

The picture has the two large floating rocks being held down by large red bricks.

 

I'm in the same situation with my planted tank and a large piece of drift wood. We were brainstorming and were thinking about drilling a core, filling it with concrete, and capping it with an epoxy of sort in order to not effect the pH. I hope that gives you an idea.

P.S. we haven't actually followed through on our idea yet.

 

Is this a marine or freshwater endeavor?

 

Is this for a saltwater or reef tank?If so,you have the wrong rock,volcanic lava rock can leech metals and toxins.But the reason it is floating is it is very lite and porus and probably has a bunch of trapped air in it still,if intent on keeping it you can tumble the rock by hand underwater helping to release trapped air.

 

After awhile your drift wood will sink. It jest needs to take in the water.

 

tie or glue it to a rock that sinks ;D

 

Sat in water for 3 months. It won't water-log.

 

I'm not sure tumbling it underwater will get it to fully submerge, but it might. Volcanic lava crystallizes very rapidly once it hits the surface, releasing gases and trapping others. It leaves all sorts of pores that can be released of air by tumbling, but others may be trapped inside its crystalline structure that can't escape (surprised I remembered something from my nap time....errr...geology class). Those trapped might cause it to float still.

 

Freshwater, sorry for not being clear.

 

Freshwater.

We've tumbled it and no air has come out at all. The air pockets are where we haven't drilled yet or deeper than we've drilled.