The best replacement for 20 volume is 30 volume.

The best alternatives to 20 volume are rated as following:
★★★★★  30 volume
★★★  30 volume developer
★★★  40 volume developer
★★  10 volume
★  30 volume creme developer

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Speaking about 20 volume, one probably shouldn't forget about bleach.

Other search terms related to 20 volume and their ratings are:
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20 volume
30 volume
30 volume developer