If your Fomaspeed N 312 prints are turning black within only a few seconds in the developer, the most likely issue is excessive exposure rather than the stop bath or fixer. Fomaspeed is a relatively fast enlarging paper, so check that the Durst M305 lens is stopped down and that the enlarger bulb is the correct wattage.
I’d also do one simple test: place an unexposed piece of paper directly into the developer under the safelight. If it stays white, the chemistry is probably working and your enlarger exposure is simply too strong. If it turns dark, investigate the developer dilution, temperature, paper fogging, or safelight. Foma confirms that Fomaspeed is suitable for tray processing with developers such as ADOX Neutol Liquid NE.
For anyone getting started with full matches replays, the same principle applies: change one variable at a time. Try a fresh sheet, stop the enlarger lens down several stops, make a short stepped test strip, and process it consistently. That should quickly tell you whether the problem is exposure or chemistry.
