As others have mentioned, it is a higher ISO that is adding the grain or rather, "noise" to your photos. As also mentioned, some camera sensors are more adapted to better noise management, but you may have guessed it, they are a lot more expensive

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There are a few things you can do, you can lower your iso and extend your shutter speed. You can also open up your apeture.
However, I am not on my home computer so I can not see your EXIF data, or rather, the details of what your camera was set to when you took the photos. Depending on your settings, you may not have that much wiggle room to extend your shutter speed or open up your apeture. Apeture is a function of the lens, the wider a lens opens, the more light it lets in, but it also affects the DOF (Depth of Field) of a photograph.
In all honesty though, I think this was a successful effort on your part. I am assuming you shot these in jpeg, had you shot in RAW, I am sure I can run them through PhotoShop and get rid of almost all the noise. It isn't difficult to do. Shooting in jpeg dramatically cuts the retained data of a file and in turn makes it more difficult to manipulate the file as you would with a RAW file. As already mentioned by someone else, RAW files tend to be much bigger. I shoot with a 24mp camera w/ a full frame sensor and my RAW files are huge. When I do a shoot and shoot off 500+ photos, that can really bog down a computer. I doubt you'd be shooting this much, but I am just giving you an example of the difference between jpeg and RAW.
If you need any more help, or if anyone in this section of the forum needs help with taking better photos, I can definitely help you all out. I have been giving advice on the Photography section here for a few weeks now, and there are a lot of people to benefit from better camera skills

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P.S., I think the best piece of equipment you can get, besides actual gear parts (Body/lens) is a tripod. In aquarium photography, you are dealing with bad lighting. I know you may think, "I have 10 Radion fixtures, I have enough light!". Truth is, that is still a difficult thing to capture because it can still be considered low light. A tripod helps alleviate some of the downfalls of aquarium photography.
I hope this helps a little.