When you buy and research a new level of fleet supply, you get a new chunk of fleet supply added to your total.   However as soon as the research is finished, you are charged the full amount of credits, metal and crystal for the new fleet upkeep level, even though you haven't built any new ships yet that actually need upkeep.   I don't think this makes sense.
 
I think this should be changed so that the fleet upkeep charges correlate to the actual ships in existance.   That way you don't have an economic penalty for maintaining ships that don't exist.
The researched upgrades to total fleet capacity would still apply, but as you take battle losses that maybe you can't replace at the same rate, your economy would actually speed up rather than be stuck at whatever fixed percentage that is determined by your fleet capacity.

While there may be a way now to reduce your fleet capacity to give your economy a boost if you get into trouble, have excess fleet capacity and can only build fast enough to maintain a lower level fleet size anyway, I don't know what it is.    But the above would be a better gameplay system anyway, IMHO.