[aaron.conway]

The first trial was 6g of hexaaquanickel(ii) dissolved in 10ml of water then 13ml of  chilled(-1C) 15M ammonia  was added. The entire solution was then chilled to -1C and left undisturbed for one hour. Instead of getting large octahedrals we just got sand < 0.5mm crystals. Any tips on growing larger crystals?

[AWK]

Large crystals grow weeks or more.

[aaron.conway]

In this situation I know it's possible to grow crystals 1cm in size within 5 hours. I just need the right techniques.

[Arkcon]

AWK: has given you a specific choice, but you can tell from his response the general rule, that slower growing crystals are larger.  Can you think of a method to grow your crystals more slowly?  One that's practical for your application?

[AWK]

In this situation I know it's possible to grow crystals 1cm in size within 5 hours. I just need the right techniques.

Then try cooling your solution with temperature gradient during 5 hours