A tank really isnt the best thing to be in! If he makes a mess in a 'regular' cage, there are lots of things you can use that arent sawdust and therefore wont make a mess...even putting a towel down is a good option, just shake it out every couple of days and wash it once a week! O0
If hes skitish, that will probably change as he gets more used to you and being handled, especially if he has had the stress of an op at any time, he will need time to chill out!
Also, maybe it could be boredom? Try putting some toys in his home, my boar harry has a teddy and a jingly ball that he loves playing with! If you look on any website under 'guines pig housing' you wont see a tank anywhere, and thats for a reason, a lot of the people on here have rescues like i do, or have lots of experience with different piggies and you can get good advice! Like you say you have done this the whole time so dont know any different, so its a good thing that people are giving you positive advice, ive made some silly mistakes that people on here have really helped me with!
Good luck with him, send us some more pictures, especially if he gets a teddy as thats super cute and he is a gorgeous piggy and certainly looks well fed!
;D
Also, a consultation with the vet is a bit over a tenner, (where i am anyway) i would and have paid that to put my mind at rest! O0
Again, no-one is being nasty we just all love our piggies and want to give you as much info as possible so you can get the most out of you little boy!
Oh yeah, make sure the bottle isnt attached too tightly as that can made the water leak!