1. don't mix tropical fish and cold water fish such as goldfish and clown loaches
2. unless you have a ton of algae, 3 plecos will starve in a small tank without proper vegetation
3. No 100 % water changes, totally shocks the fish
4. acclimate your fish in the bag you bought them in for at leat 10-20 minutes, adding tank water to the bag every 5 minutes
5. Do not overfeed your fish
6. if you have an undergravel, you're vacuuming the gravel correct?
it would really help if we had your water specifications such as Ph, hardness, nitrate, ammonia, temp etc.
did the clown loaches beat the shark up?
It sounds like you guys got in over your head pretty quick.
We are rehoming all the fish that shouldn't be in the tank, we have been in touch with the LFS who have agreed to take the fish. Although the 3 plecs are too big to all be in this tank they have all grown very quickly and don't seem to be starving, the LFS are taking all 3.
We have now learnt that we don't do 100% water changes, but we were informed to do this by another shop, seems the shops don't know that much after all.
New fish are always floated in the bag for 20 minutes, the fish are fed every other day.
Vacuuming the gravel? is that with the pipe? if so yes the gravel is vaccumed.
Did the water tests yesterday, the results are as follows

PH - 7.0
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 160ppm
I am assuming the Nitrate is whats causing the problems.
Maybe we did get in over our heads, but we started off with a goldfish and was given the tropical by a friend.
We have now purchased a larger tank, so i am just trying to do a bit of research for where we start, if anyone has any advice, pointers or links to help it would be appreciated.