Welcome to FishTankForums Theo!
First off, check out this article for future reference: Click Here
Your tank is way
too small for a single Blue Tang, much less two. If they are babies then you are ok while they are juveniles, but they are going to grow fairly quickly if fed properly and you will soon need to get them into a larger tank.
If they are not juveniles then you have two choices here: 1) Remove them now while they are alive, or 2) Remove them later when they die. Death is assured if you keep them in the tank. These guys need a bare minimum of 55 gallons and the common recommendation is 100 gallons, and both sizes are only suitable for 1.
I have a Blue in my 55 gallon and I know how voracious they are when it comes to feeding. Waste production is also high with these guys. You need to provide them with a good diet of both meaty foods and seaweed and/or frozen herbivore preperations.
I can confidently say that your overstocking practice is what doomed your goby, and is the cause of the brown algae growth. The tank is too small to play host to such a wide diversity of life. These tanks are mainly meant for either FO, or Reef systems with maybe one or two small fish in the reef setup.
Reef systems get much harder to handle when fish are kept along with them, and they are best handled once you have a solid grasp of what you are doing.
Algae is generally the result of excess nutrient availability in the tank (usually nitrate) coupled with inadequate maintenance practices. It is also a natural part of the Cycle and will occur in all new marine tanks undergoing the establishment of natural biological filtration. See Here: Click Here
Water needs to be changed out at least once every two weeks, with every week being more appropriate. Change 15% of the overall tank volume. In your case though I would step up the change to a minimum of 30% weekly in order to keep nitrate levels suppressed.
I don't think your clam is going to fare too well in the rapidly changing conditions. Be forewarned that you may lose him. It is not a good idea to try your hand at clams until your tank is mature and stable, which generally means no less than a year old.
There is no way for me to know the full extent of the situation unless I know more of your filtration capabilities and tank parameters. Follow the link provided above and also let me know the following:
- How much Live Rock do you have in the tank?
- How much sand do you have in the tank?
- Did you properly cycle the tank before introducing your livestock?
- What lighting do you have over the tank?
You can find the answers you need here or we can point you in the right direction if needed. Let us know if you have any other questions.