Start with what the tray is showing you.
Use the symptom you can see to narrow the variables worth checking first. Most problems are easier to diagnose when you separate moisture, light, airflow, seed density and sanitation instead of changing everything at once.
What are you seeing?
Choose the closest match. Each guide explains the first things to inspect before you start changing the whole setup.
Mold or root hairs?
Learn the location, texture and moisture clues that help tell them apart.
Leggy seedlings
Check light timing, intensity, blackout length and canopy competition.
Uneven germination
Review seed quality, sowing density, contact and moisture consistency.
Watering problems
Use tray weight, drainage and root establishment to guide watering.
Airflow & humidity
Separate circulation problems from room-level humidity and condensation.
Clean the system
Remove residue first, then sanitize trays and tools appropriately.
Stop the conditions that favor mold
Manage moisture, airflow, sanitation and density as one system.
Check the whole setup
Compare countertop, shelf and rack systems when problems are structural.
Food-safety decision
When a tray is clearly moldy, foul-smelling or otherwise questionable, be conservative.