!!!!??? It's happening again!

[Reef Pilot]

Well, your extreme actions, overdoses, etc, can't have been good for your tank. And if I remember correctly, you had your bio pellet reactor running offline for a while where it could have built up a lot of carbon from the pellets. Then the bacteria died, causing hydrogen sulphide. I believe you also discharged that into your tank, which then could have caused a die off in your live rock, which is still leaching out, and maybe causing your current issues.

So, a whole lot of things that could be causing your current problems. I think the old rule of not making drastic changes too quickly would apply here. That is especially important when using bio pellets or carbon dosing.

I used Chemiclean once (and followed instructions precisely) about 3 years, and it did the job well for me then. Have not needed it since, as I dose MB7.

And should test for nitrites too. If they also are high, and nitrates low, then you indeed are having a cycle. But with a 2 year running tank, and with all the die off now, would expect your nitrates to be high instead.