Provided they are not contaminated, they will help. Give them a good scrub under the cold water tap. Coral, shells etc are calcium carbonate so they will increase both calcium and carbonate, but they won't increase hardness enough by themselves.
Earlier in this thread you state the GH is "really bad" but you do not give the number. This is very important because it is the starting point to fix the issue.
Coral and shells were mentioned...yes, over time these do dissolve very slowly, adding calcium primarily and this raises GH. However, it is not a significant amount, depending upon the initial GH. If the GH for example is very soft, you are not going to help by adding calcareous items like coral and shells. But we need to know the GH (and pH) of your source water.
Light was mentioned a few posts back...yes, turning the light on wil usually cause some sort of reaction in fish because it is a shock, and turning the light off can do the same. That in itself is not the issue here, but it is worth remembering that when the fish is stressed out and fighting off something, these additional stressors donot help.
Coral and shells were mentioned...yes, over time these do dissolve very slowly, adding calcium primarily and this raises GH. However, it is not a significant amount, depending upon the initial GH. If the GH for example is very soft, you are not going to help by adding calcareous items like coral and shells. But we need to know the GH (and pH) of your source water.
Light was mentioned a few posts back...yes, turning the light on wil usually cause some sort of reaction in fish because it is a shock, and turning the light off can do the same. That in itself is not the issue here, but it is worth remembering that when the fish is stressed out and fighting off something, these additional stressors donot help.
I would say you need to get to the store rather quickly to get those salts if you don't want to see more die off. Also - you don't mention what the ppm of Ammonia there are in your tank - that's probably the most toxic substance in your tank, but if you are adding Prime every 48 hours you'll likely keep the ammonia in the state of non-toxic Ammonium - you still want to get rid of it but it won't kill your fish. Since your water has likely been hard from the beginning I'm not convinced that's what's causing the deaths - you'd think you would have seen it a long time ago. You also don't give real numbers other than nitrates and nitrates are "good" but hardness is "real bad" - can we get the actual numbers so we can judge what is "good" and what is "real bad"? That sometimes depends on the species of fish.
In general - what is your animal husbandry practices - how often do you do a water change and what % of water do you change? Is the gravel filthy - do you clean your gravel? What other media besides carbon do you have with your filter? Most people use some type of ceramic biobeads for the "good" bacteria to live on, others use sponges - some use both. Did you cycle your tank before adding fish (I'm guessing you didn't since you didn't have a test kit). Some or all of this can eventually come to a head - and cause a sudden die-off - but it sounds like for most of the period of time you've had your fish the water has been at least suitable enough to keep fish alive. I would bet mostly on not frequent enough water changes (such as 50-75% per week ideally) may have caught up with you.
Another thing to consider is not having enough oxygen circulating in the tank - if you don't do frequent water changes that can cause it, the filter itself can help oxygenate the tank, as can plants and the "bubblers" - that could be the source of "heavy breathing". So try a big water change if you haven't done one recently. Most fish enjoy having fresh water. Some of mine even get quite exited about it! When you have "bad" water it can eventually lead to all sorts of diseases - which it sounds like you have so I'd say let's get the water straightened out first, and then post the values for Ammonia, Nitrites, Nitrates and PH. If you have the actual water hardness numbers (different kit) then you can post them or you can go to your city's website for their water department and hopefully look up the values your water department has measured for your area.
In general - what is your animal husbandry practices - how often do you do a water change and what % of water do you change? Is the gravel filthy - do you clean your gravel? What other media besides carbon do you have with your filter? Most people use some type of ceramic biobeads for the "good" bacteria to live on, others use sponges - some use both. Did you cycle your tank before adding fish (I'm guessing you didn't since you didn't have a test kit). Some or all of this can eventually come to a head - and cause a sudden die-off - but it sounds like for most of the period of time you've had your fish the water has been at least suitable enough to keep fish alive. I would bet mostly on not frequent enough water changes (such as 50-75% per week ideally) may have caught up with you.
Another thing to consider is not having enough oxygen circulating in the tank - if you don't do frequent water changes that can cause it, the filter itself can help oxygenate the tank, as can plants and the "bubblers" - that could be the source of "heavy breathing". So try a big water change if you haven't done one recently. Most fish enjoy having fresh water. Some of mine even get quite exited about it! When you have "bad" water it can eventually lead to all sorts of diseases - which it sounds like you have so I'd say let's get the water straightened out first, and then post the values for Ammonia, Nitrites, Nitrates and PH. If you have the actual water hardness numbers (different kit) then you can post them or you can go to your city's website for their water department and hopefully look up the values your water department has measured for your area.
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