Im running out of excel too. Well have to look for the biggest bottle and order some. I have a empty 2 liter of excel. I wonder how big it comes in. I think they also offer Prime etc for ponds that are a large size and I think it is the same stuff.
10 gallon hospital tank.
29 & 50 gallon - High Tech C02 enriched.
50 gallon Pond - Natural.