Zebs get to about 70-80 cm maximum, snowflakes a tad smaller at about 65-70 cm. A 4x2x2 would be about the minimum for any small eel species IMO. Theres plenty of advice that contradicts this on the net saying from 50gal (200L or standard 4fter) and above. However you need to take into consideration that they are messy eaters and produce lots of waste thats high in nutrients, meaning a 420L or there abouts (a 4x2x2) would suit them better, as the water parameters keep more stable, and are able to cope with the sudden expenditure of waste products (eels are usually feed 2-3 times per week, sometimes less). As long as you have an efficient skimmer running on your system, and a low stock load already living in it you shouldn't have too many problems