What are the dimensions of the tank?
You should be fine filling the tank only partially, you just have to keep in mind how many gallons you're actually filling it and don't overburden the tank with too many axxies (it may be a 40 gallon, but if you only fill it halfway you only have 20 gallons of water).
I was going to do that with my 20-gallon but ended up filling it up to the top to accommodate my filter which was too high, so it depends on the type of filter as well. Internal or sponge filters should be fine which HOB filters without some extensions might not reach the water level (my initial problem) and will cause quite a current with how far the water would have to fall, so just a few things to keep in mind.
I don't have personal experience yet with how much or little axxies care to swim great distances to the top (I'm still very very slowly cycling my tank), but it may not be a problem to fill it up to the top.
