Lava Rock And Phosphates Problem?


New Member
Jul 16, 2017
Hi There,

I am not able to keep my phosphates level. In a matter of a couple of days a PO4 dose of 0.5ppm drops to non measurable values.

All the plants are struggling with deficiency symptoms mostly on the older leaves. For sure they are not consuming a lot of phosphates, first because there is not yet a lot of plants and second because they are stunting.

It is a low-tech 120L tank with liquid carbon (normal recommended dose) and RO water. I have never had this problem in the past. The only things that I have done differently this time is:
- to use quite a bit of lava rock to support the substrate for the dragon stone aquascaping. At a rough guess,I have used about 15 Kg of lava rock
- My eheim Prof 4+ 350T is fully packed with a lot of bio-media (susbstrate pro by eheim) as I am using a pre-filter for the mechanical filtration. It used to have far less bio-media (the one that comes included when you buy it!) in my previous set-up.

I have easy growing plants medium density, ADA soil and moderate light. Very,very standard and simple set-up...

I keep these values quite constant without problems apart from the PO4::
KH 2
Ca= 30 ppm
Mg=10 ppm
NO3= 10-15 ppm
(PO4=0.5 ppm)
Fe: 0,2 ppm
K= 20ppm

and I also dose all the other traces moderately and I perform 50% weekly water changes.

Can it be the lava rock??

Thanks for sharing your thoughts.