Yeah use any old nails with the ply, the ply stops it from pulling through make sure there are 2 of you to help get things setup before nailing especially when doing the roof, also make sure you know where your studs are in case you want to attach anything solid I would suggest marking the poly top/bottom with a vivid line or something where the studs are.
I used standard silicone from mitre10 the cheapest stuff and it worked fine, it actually takes stuff all time to do all your joins lick your finger and then smear it all off probably just as long as taping it by the time you tangle the tape up a few times and have to stick a few layers on to cover the join or where you go on an angle lol trust me I have done it on 2 rooms now ended up ripping it all off (what little was still stuck) and siliconing it.. I haven't found any tape that sticks to poly long term and it looks ugly as well, I was using it and then using a stable gun to hold it on as well until someone mentioned the silicone which works out really well.
I had condensation issues behind the poly in my first room so ran a dehumidifier in the second time, when I pulled that one down it was perfect underneath so I would recommend that, it also makes the room warmer. I run a dehumidifier full time in my room on its "normal" setting plumbed to drain constantly outside. I also tried to have half descent lids on all tanks and things to reduce the amount of water that was evaporating.
As I said I have built 2 fishrooms using these methods and these are the things I found to improve them, my second room was alot more efficient than my first using the things I have mentioned above.