Decant arbitrage. Potions of the same type but different dose sizes often trade at different prices
per dose. Because decanting is
lossless (three 4-dose = four 3-dose), you can buy the cheapest-per-dose size, decant, and sell the richest — pure profit on the gap, after the 2% sell tax. Decant for free at
Zahur (Nardah) or
Bob Barter (Grand Exchange, members), 5 gp per extra vial when splitting down. Profit is per potion you
sell; it's capped by buy limits and your decanting time, and prices are live so margins move. See the
decanting guide. Not affiliated with Jagex; not financial advice.