I normally use crushed coral. If you need a quick fix baking soda works great.
The fist thing you need to understand is that pH crashing does not stop the cycle, it just can appear this way. It works at pH 4.2. I have cycled a tank at that level. But do not take my word for this.
Gieseke, A., Tarre, S., Green, M. and De Beer, D., 2006. Nitrification in a biofilm at low pH values: role of in situ microenvironments and acid tolerance.
Applied and environmental microbiology,
72(6), pp.4283-4292
ABSTRACT
The sensitivity of nitrifying bacteria to acidic conditions is a well-known phenomenon and generally attributed to the lack and/or toxicity of substrates (NH3 and HNO2) with decreasing pHs. In contrast, we observed strong nitrification at a pH around 4 in biofilms grown on chalk particles and investigated the following hypotheses: the presence of less acidic microenvironments and/or the existence of acid-tolerant nitrifiers. Microelectrode measurements (in situ and under various experimental conditions) showed no evidence of a neutral microenvironment, either within the highly active biofilm colonizing the chalk surface or within a control biofilm grown on a nonbuffering (i.e., sintered glass) surface under acidic pH. A 16S rRNA approach (clone libraries and fluorescence in situ hybridizations) did not reveal uncommon nitrifying (potentially acid-tolerant) strains. Instead, we found a strongly acidic microenvironment, evidence for a clear adaptation to the low pH in situ, and the presence of nitrifying populations related to subgroups with low
Kms for ammonia (
Nitrosopira spp.,
Nitrosomonas oligotropha, and
Nitrospira spp.). Acid-consuming (chalk dissolution) and acid-producing (ammonia oxidation) processes are equilibrated on a low-pH steady state that is controlled by mass transfer limitation through the biofilm. Strong affinity to ammonia and possibly the expression of additional functions, e.g., ammonium transporters, are adaptations that allow nitrifiers to cope with acidic conditions in biofilms and other habitats.
from
https://journals.asm.org/doi/full/10.1128/AEM.00241-06
There are plenty of studies that confirm the above if you need to see more.
What causes the the pH to crash during a normal cycle is the cycle itself which is acidic. What normally prevents this are the carbonates and bicarbonates which make up KH in aquariums.
Instead of my having to reproduce the info, the OP should go here and read
https://fins.actwin.com/mirror/begin-chem.html#reference
It will pretty much explain it all in pretty simple terms. Of course the info is somewhat dated and a few things are off but, on balance, it is good info and where I started two decades ago.
