You can try Fauna Marin Power Trace 1-4. This is available from either MarineLife (only 250ml bottle in stock) or Iwarna (both 250ml and 500ml bottles in stock). The dosage is a maximum of 10ml/100L per week. This dosage can be lower depending on your nutrient concentration and your amount of corals. For your JBJ 28 Gallon tank, your maximum dose will be about 10ml per week after taking water displacement by rocks, sand, etc. You start with around 2.5ml per week and slowly increase the dosage while observing how the corals respond. So a single 500ml bottle will last you more than a year. That's about $11 per month on trace elements if you get Power Trace 1-4 in 500ml bottles.
Dosing frequency simply means splitting up the dose needed to maintain your Ca/kH/Mg levels into many smaller doses. For example for my 5 gallon pico tank, I am currently dosing 16ml of Part A & 16ml of Part B a day. I split this dose into 24 dosing intervals, meaning each dose at any one time is around 0.66ml. If I only set 4 intervals to dose this amount, each dose will be 4ml. 4ml of sodium bicarbonate into 5 gallons of water at one shot will cause a sharp rise in kH and such shock to corals do not promote optimal growth.
You can opt for Balling Light Method, the trace elements are added in to you Ca/Mg/kH solutions and you do not have to dose the trace elements individually anymore.
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