Aquarium issues...

[foxtrotblues]

i have a few items in my aquarium, like backgrounds, that i recently received but am unable to select. all i have in said aquarium are two of the three starter fish- the goldfish and the pointy one. i can't put the other goldfish in for some reason, nor can i put in the background i just received from the daily chance- please help me, my fish deserve better!  

[Sharkworld]

The only thing I can assume is you are trying to place them in your aquarium from your main inventory rather than the fish tank inventory.
If you visit your aquarium you will find a drop down arrow in the top left hand corner. This will open up your aquarium inventory. You then go to the relevant section backgrounds/fish/decorations etc and drag and drop the item into your tank.
If however this is what you have been trying to do and are still not having any success you may need to contact a mod to get your tank fixed.