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Lava rocks - inert enough?

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I purchased some large black lava rocks that I intended to use in my 40G breeder Caridina setup from the following:  http://www.ebay.com/...=item3f49e684a5  


I boiled the rocks in tap water, rinsed for 24 hours in RO, drained, then refilled about 2 gallons of RO (pH ~6.4) to cover the rocks in a 5-gallon bucket.  Over the last 6 days, I've see the TDS take an initial early rise, then drift upwards each day (albeit slowly).

  • Start - 4 ppm TDS
  • @12 hours - 19 ppm
  • @4 days - 38 ppm
  • @5 days - 44 ppm
  • @5.5days - 49ppm

I know that many of you use smaller lava rocks and consider them inert for shrimpkeeping purposes........was hoping to solicit some opinions on these rocks and their use.  

  • If I'm planning on using these rocks in a 40-gallon setup, would the TDS rise be diluted to the point that it isn't of concern?  
  • Seeing as how I'm in the initial stages of cycling and don't plan on adding shrimp for 30-60 days, should I go ahead and add the lava rocks and monitor their effects over the next month?  Or would it be better to continue to soak and monitor the rocks in the 5 gallon bucket until making a decision?
  • Should I be testing for anything additional that could be of concern "leeching" from deposits with the stones?



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Thats kind of odd the TDS is increasing, I use lava rocks from Home Depot for barbecues and I have never noticed any TDS change in my tank. The only thing I do to prep the rocks is rinse them really well. Im sure Meta would have some specific insight into this topic.

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No matter what rock or substrate you use the TDS will rise. With the possible exseption of a pure qruartz based rock with no inclissions. Also yes the larger the tank the less impact it will have in raising the TDS. I life TDS worked on a linier scale with a TDS of 49 in a 5 gallon bucket assume 4 gallons. Then in your 40 it would be 1.9. Now this all assumes you add no additional tds' s.

Have you seen any changes in your ph, kh or gh yet.

have a gneiss day

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Have you seen any changes in your ph, kh or gh yet.


Good question.  I had only been monitoring TDS, so I'm not 100% sure of my starting points, but can make some safe assumptions.  I started with 0 TDS RO water, pH 7 (pH 6.4 when off-gassed for 24+ hours).  I've checked all three parameters just now and pulled the following results:

  • pH 8.0-8.2
  • GH 1-2
  • KH 2

Assuming my starting RO had 0 GH and 0 KH, I can understand that anything alkaline would cause the upward pH swing.  That said, there's clearly something in the rocks that is causing all three parameters to rise.


Coincidentally, I'm in the process of cycling my tank and am having to adjust pH upward because the buffering capacity of Controsoil........I was originally at pH 5.96.  It actually could be helpful to add these rocks as they would slightly increase both pH and KH, which could reduce my need for baking soda........thoughts?


I'm really at a loss and trying to decide if I should continue with my "testing", restart my "testing", or add the lava rocks to my cycling tank and see what shakes out.  I have constant pH monitoring (Pinpoint) that would allow me to have a clearer picture of the effects of the rocks, and could do GH/KH testing several times throughout the day if so needed.


Any advice is welcome, interested to hear peoples thoughts.

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I bought three orders of the black rock from that seller and use them in several of my shrimp tanks.  They passed the muratic acid test - and they don't leach anything at all after initial rinsing to clean out the pores.  Shrimp love to pick at the rocks as "stuff" will live in the pores.  

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