David K. Bradley;4244799; said:
Manlyfish:
In the past when I have thought of owning a particular fish or a combined number of fish, I was advised to look at the full grown adult size of the fish in inches. Then calculate about 1.5 gallons in water volume per inch of fish.
As an example, the adult size of say 9 fish who could be kept in a comunity tank. Lets say 3 @ 10 inches, 3 @ 12 inches and 3 @ 14 inches. So we calculate 3x10 + 3x12 + 3x14 = 108 inches of fish being full grown. Next we calculate 108 inches x 1.5 gal. of water volume = 162 gallons of water volume minimum to house all 9 fish. This is how I was instructed on how to calculate my tank size for fish I wanted to have.
So, even if the fish you mentioned above are small now, they eventually outgrow you 75 gal tank. IMHO, I would get a larger tank if you plan on having those fish mentioned in your question. Just calculate their adult size and then calculate the size of tank you'd need for your fish. Don't forget about the bottom dwellers as well if you plan on having them too, as their growth needs to be calculated in the calculations too.
I hope this was helpful information to you. Good luck with your 75 gal. tank.