Solo 401K contributions investing activities

[fjpuentes1974]

I am making decent profits from my investing and trading business, on top of my W2 from my day job.

 Can I open a solo 401K from my investing activities? is this considered a pretax contribution ?

[dmertz]

Contributions to a solo 401(k) would need to be supported by net earnings from self-employment.  You are self-employed with respect to your personal investing activities only if you qualify as a "trader."

https://www.irs.gov/taxtopics/tc429