He was speaking on his return here from Iran where he and his British and French counterparts won an agreement by Iran to open its nuclear programme to snap inspections.
They were "very difficult negotiations", Fischer told reporters but he added that they had been "impregnated here and there with realism".
"Everything depends now on the implementation" of the accord which he said "offers a great opportunity for Iran and that should not be missed".
While making no concessions, the three EU foreign ministers had tried "to open to Iran the path of return into the international community", he added.
Iran announced earlier it would sign an additional protocol to the nuclear Non-Proliferation Treaty which will allow the Vienna-based International Atomic Energy Agency (IAEA) to carry out surprise visits to suspect facilities -- a key tool if Iran can ever be given the nuclear all-clear by the watchdog.
Tehran also pledged to show "full transparency" to the IAEA, reiterated its commitment to the NPT and vowed atomic weapons had "no place" in its defence doctrine.
It also said it would suspend all uranium enrichment and reprocessing activities.
The decision came just 10 days before the expiry of an IAEA ultimatum for Iran to come clean about is nuclear programme.