If your Fomaspeed N 312 prints are turning black within just a few seconds in the developer, the most likely cause is overexposure 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 carry out one simple test: place an unexposed piece of paper directly into the developer under the safelight. If it remains white, the chemicals are probably working correctly 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 NEUTOL.
For anyone starting out with full-match reproductions, the same principle applies: change one variable at a time. Try a fresh sheet, stop down the enlarger lens by several stops, make a short stepped test strip, and process it consistently. That should quickly tell you whether the problem is exposure or chemistry.
