Hey there
@SamuelW! I agree that water looks very murky, I haven’t seen even the worst diatoms make water that brown. Have you done any water changes at all since cycling your tank? As mentioned, I think the first step here is to run a test of all parameters to get a baseline. I also think regardless, now is a good time for a water change and by the looks of things and with how small the tank is, I’d suggest a 50% change, but test BEFORE doing the water change, then test again 48 hours later to see how the tank’s responded. Did you use old dry rock in your tank? I’ve seen water look like that when dry rock hasn’t been rinsed before use and the dead organics clouded the water. Last bit of advice, I would suggest testing your parameters
frequently (minimum once per week) until your tank is well established. It’s a lot easier and more likely you’ll be successful if you can catch things and address them early on rather than wait to test once things are going south.