I use the suction cup glass thermometers and they seem to work well, as for buying another tank, two or more would be the way to go as there is no such thing as to many, just not enough room. You could have some guppies, swordtails, platties or mollies are peacefull, there are a few types of loaches that are good in a community tank e.g. clown, striata, yo-yo, queen, zebra, (stay clear of burmese tiger loaches, they are agro) a couple of otocinclus/corys/ b/nose as a clean up crew. Thats just to name a few. Check out your LFS as there are bound to be some tropicals that take your fancy : -)>