I just found a $9.95 Lux meter on sale at Amazon (US). It seems to work really well. Linking to Amazon doesn't always work here, but if you search for “Proster Digital Luxmeters” you should find it.
It measures light levels - and reports that in either Lux or Footcandles. That isn't as good a a PAR measurement, but it's better than just eyeballing it.
It looks like they only have four units left (not sure if they will restock). The regular price is $19.95, still a good price.
While I haven't tried it yet, this unit is the size of a cell phone. I have a clear cell phone bag to keep water off a phone (at the beach, or while swimming). Putting this in a bag like that and submerging it in your tank would give you light readings at depth, and in different locations.
I'm using it to test the grow lights over my indoor Bonsai trees. It can also be used to compare one light source to another. As I said though, it does not measure PAR, so the total light reading, if the wrong wavelength, won't tell you too much.....