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Try a clothes peg as a temporary solution. Crimp the hose just enough to slow the flow rate down.

I mean ... remineralise to a suitable TDS, GH, KH level for the type of shrimp you keep in the tank. I'm not saying you are doing it wrong. That's why I suggested changing most of the water. We don't know what is missing. So safer to just reset it.

Unfortunately i didn't receive an order for an air flow regulator to for my home made drip acclimater that i ordered the day before i left so i cant do slow water changes, i was thinking of trying my usual method of 2 liters of water per day until solved... im thinking of remineralising with 400TDS so i could theoretically bring the TDS down over time that way. i'll see what that does over a week and if nothing changes ill dose with nitrite munus im planning on putting the existing tank in the living room as a cull tank and offering them to people on gumtree for collection. i think the living rooms acceptable for them to view the animals and select. i dont know whats brought up my GH other than maybe the mineral stone but yeah im removing that when i aquascape later today. sound reasonable?

The million dollar question? I guess that is where the specific shrimp products come in, Gh+ that you use with water changes, and they must have been developed to give the shrimps what they need. Wow, 30% of water in a 220L tank is a lot of water, I don't even want to think about lugging all that water about? I would guess that that is sufficient to top up the minerals each time, I would certainly hope so anyway! If you have 500 shrimps already in the tank then the tank may already be at/close to its maximum occupancy? The shrimp haven't been in this same tank for 3 years though have they? What was in there before? I assume it has had RO remineralised water in there for the 3 years? Simon

A 3yr old tank, without additional minerals, has probably run out of something, especially if the shrimp have to compete with the minerals plants absorb. Try a complete water change and remineralise it properly. That means you will need to reacclimate the shrimp.