well they seem to be doing fine in the salt i have whats the differnce
They won't live out their natural life expectancies and will be more prone to getting diseases and stuff if you don't use the correct type of salt. You shouldn't really be keeping these fish in the same tank as your freshwater fish as it is not good to dose freshwater fish like catfish with salt in the long term as it puts a great strain on their osmotic processes and could eventually lead to kidney failure in the fish.
The silver dollars will grow too large for your tank and should not be kept singularly anyway, ideally they should be kept in at least a 50gal as they can grow up to 5inches long each and should be kept in shoals- you also need to feed them the right diet too as they are herbivorous characins;
[URL="http
/www.fishforums.net/content/Characin.../Silver-Dollar/"]http
/www.fishforums.net/content/Characin.../Silver-Dollar/[/URL]
Figure 8 puffers are semi aggressive and they may pick on the silver dollar, they may also fight between each other;
[URL="http
/www.fishforums.net/content/Oddballs...igure-8-Puffer/"]http
/www.fishforums.net/content/Oddballs...igure-8-Puffer/[/URL]
You say you got sold a pacu not knowing how big it grows, but you will continue to make yourself vulnerable to buying unsuitable fish for your tanks unless you research fish more thoroughly before you buy them- at the end of the day, pet shops don't have a duty to tell you everything you need to know about the fish you want to buy, and to be honest most pet shop staff tend to have pretty limited knowledge on the fish they sell anyway (since its not something that is really covered by their basic training in general). So really its up to you to make sure you know all you need to know about the fish you are buying and it is your duty as a responsible fishkeeper to research fish properly before you buy them

.
You need to ID your catfish as there are thousands of types of bottom dwelling catfish, some only grow to an inch long while others grow to over 4ft long. It does sound like you could have a cory or a pim or a pleco though, does the catfish look anything like this;
[URL="http
/www.fishforums.net/content/Catfish/...ered-Corydoras/"]http
/www.fishforums.net/content/Catfish/...ered-Corydoras/[/URL]
Or this fish;
[URL="http
/www.fishforums.net/content/Catfish/41743/Pim-pictus/"]http
/www.fishforums.net/content/Catfish/41743/Pim-pictus/[/URL]
Or this fish;
[URL="http
/www.planetcatfish.com/catelog/speci...?species_id=148"]http
/www.planetcatfish.com/catelog/speci...?species_id=148[/URL]
???
Dosing the aquarium with salt on a regular basis will be doing nothing beneficial for your fish, although salt can be used to treat certain diseases and parasites, it should not be used on healthy freshwater fish as it will put a strain on the fishes kidneys etc in the long term (and it certainly won't improve their immune systems). However now you have these puffers, you do need to do something about creating a brackish environment for them if you are to look after them properly- i would suggest setting a 20-30gallon brackish tank, fishless cycling it (
[URL="http
/www.fishforums.net/content/New-to-t...shless-Cycling/"]http
/www.fishforums.net/content/New-to-t...shless-Cycling/[/URL] ) and moving the puffers into

.