Plant issues

[roadhead303]

I'm running into issues 3 1/2 weeks in.

PPM 1400
PH 5.9
Water temp 66
12/12 lighting
Sealed room
Temp 78
Technaflora nutrients

I just discovered my Co2 regulator was off for the past 3 days. Could this be the issue? The furthest plant in the corner is the only one showing issues. Could the C02 be causing this?

 

[Seamaiden]

I've never run MPBs, but that looks like a classic Ca/Mg problem to me. This should help bump you up again.

Interveinal chlorosis usually = Mg
Necrotic lesions at leaf margins = Ca

 

[alfonso2002]

Your good Seamaiden