Don't completely clean the tank. This would be very bad for your tank because you will lose your cycle. I don't think that bacteria is the problem here. Could you test the tank and post all of the figures? Ammonia, NitrAte, Nitrite and PH? Once we better know these numbers, we can advise you.

The only things I could ask with your stocking of prior tanks is how long did the fish live for and how long did they get? Some people say that goldfish will grow to the size of a tank, but this is a myth. The reason why some stay small is because they are stunted from living in such small waters. Goldfish can live for over two decades if cared for properly, many living several years and attaining large size. If the fish didn't get big...and didn't live for several years (or more), then this stocking situation was not a success. A dophin could probably survive in a tank the size of your bedroom, but it wouldn't thrive. The same thing can be said for goldfish and other fish that shouldn't be in small tank situations.

Iridescent Sharks are actually a type of catfish from Thailand. They can reach in excess of FOUR FEET in size when full grown and weigh up to a HUNDRED POUNDS. They shouldn't even be for sale in the fish trade, let alone to live in a small fish tank. You would need a tank thousands of gallons big in order to simply properly house them. The babies that they sell in stores are quite cute, and they all succumb to the same fate of dying deaths of lives that are much shorter than they should be...and are severely stunted in growth because of having to deal with small tank/water parameters. There are many fish sold in common pet stores today that shouldn't be. Common suckermouth catfish and plecos are in a similar boat. They get far too big for small aquaria and should be kept in large tanks. While they certainly don't get as large as the Irid. Shark, they still need to be considered for their ultimate size.

We can help you get through the transition period to a larger tank, but if you don't plan on getting one, then the goldfish and the pleco will need to go back to the store. A 10g tank is simply not large enough for them.