lost an agelfish last night

hello,
I had a pair of angelfish only 2 1/2 years old in a 46 gallon tank, and
unfortunately lost one this morning (male). they were both about 6 inches in
length and seemed very healthy. all my water parameters were all in check. I
perform 25% weekly water changes and feed them twice a day. I'm really not
sure what happened, there were no visible signs of cause of death so I am a
bit confused.

anyway, I know angelfish are very picky as to other angelfish tankmates. I
would like to get a replacement angelfish, but I am afraid it wont live long
with a full grown angelfish in the same tank. should I get two medium sized
angels or just one medium size angelfish?

any advice is appreciated!!

Thanks,

James