Capped sand w/gravel and lost 10 fish

[Just Add Water1]

I have a 20 long, it’s been established for about 2 years. There are mesh bags at the back filled with Fluval Stratum coved with black sand. There are live Amazon Swords rooted in them. The rest of the tank is about 1-2 inches of black sand.
Recently I decided to cap the sand with black aquarium gravel as 2 years of vacuuming had depleted my sand layer to about half to one inch depth. Since adding the gravel I have lost 6 very established fish and 4 newly added fish.
I do 50% water changes every other week and have for years with no issues.
Before adding the gravel I vacuumed the tank as usual, washed all gravel and used water conditioner in the final rinse before adding it to the half full tank by hand gently as to not stir up the sand.
Have I made a grave mistake in capping my sand?

 

[gwand30 146]

I have a 20 long, it’s been established for about 2 years. There are mesh bags at the back filled with Fluval Stratum coved with black sand. There are live Amazon Swords rooted in them. The rest of the tank is about 1-2 inches of black sand.
Recently I decided to cap the sand with black aquarium gravel as 2 years of vacuuming had depleted my sand layer to about half to one inch depth. Since adding the gravel I have lost 6 very established fish and 4 newly added fish.
I do 50% water changes every other week and have for years with no issues.
Before adding the gravel I vacuumed the tank as usual, washed all gravel and used water conditioner in the final rinse before adding it to the half full tank by hand gently as to not stir up the sand.
Have I made a grave mistake in capping my sand?

Did you loose the well established fish before or after adding the new fish? Did you quarantine the new fish for 4-6 weeks before adding them to the tank?

 

[Just Add Water1]

After and no I did not quarantine them. I understand that can be a reason for loss, but I’m specifically asking if I harmed my tank by capping my sand before I go tearing the whole thing apart to remove the new gravel.

 

[Tomsfish10 53]

It does not sound like the way you added the gravel that it would be the problem.

 

[TwoTankAmin25]

Did you test the tank for ammonia as you started losing the fish? Normally some amount of the bacteria will be in one's substrate. So you capping the sand may have caused the bacteria living on the top 1/2 imch or so to be deprived of oxygem and this one of the things they need. The Plants will also use ammonia. But, an amazon sword is a heavy root feeder and also these will grow to about 3 time the height of a 20l. I had one outgrow a 75 gal.

Also, you do not indicate the what the 6 fish were nor how big nor did you do so for the 4 fish you added. Adding fish can cause an ammonia spike depending on the size of the existing fish v.s. the newly added ones.

In order to help you know what went wrong it helps to have a lot more info than you have provided which makes it hard to figure out what might have gone wrong. Things like water parameters, specific fish involved. dechlor used and anything else added to the tank all might be involved but we have no clue to any of this.

And or course tythe new fish might have been the cause. Information on how the fish were behaving before they died. How they dies matters as well, Did they all day in a short period of time or did they die a few at a time. More information is always better than too little when it comes to trying to diagnose what went might have gone wrong.