Its going to produce foam, so dont top it off now. As long as its producing CO2, you dont have a problem. If you bulk age in a carboy after fermentation, you could top it off, but IMHO it is better to just blow some C02 on the top to keep the oxygen off it. If you dont have CO2, you can get a can of wine conditioner from the wineshop that is the same thing. For that matter, keeping the oxygen off is really only an issue if you want to age this for a long time
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