Try not to airate the beer. Once fermentation has begun, you risk the chance of oxidizing your beer! You can gently swirl your fermenter around to see if you can suspend the yeast some. Larger beers such as what you brewed take longer to ferment out. What recipe and yeast did you use? Also, did you make a yeast starter? Thats a rather large beer to not have one.
I made a liter starter pured into 1 gallon starter. Pitched the whole thing after 18 hours.
I have never used yeast nutrient in my life, and knows little about its purpose.
This is the recipe I attempted. Fermented at 17 degrees C:
Recipe Specifications
--------------------------
Batch Size: 18,93 L
Boil Size: 22,71 L
Estimated OG: 1,091 SG
Estimated Color: 20,2 EBC
Estimated IBU: 39,0 IBU
Brewhouse Efficiency: 75,00 %
Boil Time: 60 Minutes
Ingredients:
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Amount Item Type % or IBU
5,45 kg Pale Malt (2 row) (8,0 EBC) Grain 74,86 %
0,30 kg Cara-Pils/Dextrine (3,9 EBC) Grain 4,12 %
0,25 kg Aromatic Malt (51,2 EBC) Grain 3,43 %
14,80 gm Goldings, East Kent [4,00 %] (90 min) Hops 5,9 IBU
23,05 gm Saaz [2,00 %] (90 min) Hops 4,6 IBU
29,30 gm Hallertauer Hersbrucker [1,60 %] (90 min)Hops 4,7 IBU
23,10 gm Saaz [2,00 %] (45 min) Hops 4,0 IBU
17,30 gm Goldings, East Kent [4,00 %] (45 min) Hops 5,9 IBU
30,05 gm Hallertauer Hersbrucker [1,60 %] (45 min)Hops 4,1 IBU
29,60 gm Goldings, East Kent [4,00 %] (15 min) Hops 5,5 IBU
30,05 gm Hallertauer Hersbrucker [1,60 %] (15 min)Hops 2,2 IBU
23,05 gm Saaz [2,00 %] (15 min) Hops 2,1 IBU
0,50 tsp Anise, Star (Boil 15,0 min) Misc
0,50 oz Coriander Seeds (Boil 10,0 min) Misc
1,00 items Vanilla bean (Secondary 2,0 weeks) Misc
0,83 kg Candi Sugar, Medium Amber (20,0 EBC) Sugar 11,40 %
0,45 kg Corn Sugar (Dextrose) (0,0 EBC) Sugar 6,18 %
1 Pkgs Belgian Witbier (Wyeast Labs #3944) Yeast-Wheat