hi and welcome first off with the saltwater look into the equipment and products vary good as if you buy crappy equipment you will prob. have to re-buy stuff so if you are looking to get something ask here as everyone here can help.
now with the fresh water most fish leave plecos alone but if...
ya a tang is too small unless you plan on upgrading your tank soon. also how old is your test kit i know if thay are pritty old thay tend to miss read you should take some of your water to your lfs and have them test it to see if its on point