If its a niger cat then you havnt got long and will need an 8ftx4ftx3ft idealy for a full grown adult even this may need to be bigger eventually.
If its a megalodoras its a big one at 18" but it wont get much bigger 24" tops and they grow at the rate of about 1" a year from 18". So your 5ftx2ftx2ft should be ok for a little while but an 8ftx30"x30" would be better
Megalodoras are my fave catfish send it my way if it is lol