Hello. I did my fluid in the game, gave it a water material and added methods:

   public boolean isFullCube(IBlockState state)
   {
       return false;
   }

   public boolean isOpaqueCube(IBlockState state)
   {
       return false;
   }

and in the constructor put blockrenderlayer
 

   public CustomWater(Fluid fluid, String name)
   {
       super(fluid, Material.WATER);
       setCreativeTab(Realism.tabRocks);
       setUnlocalizedName(name);
       setRegistryName(name);
       
       setRenderLayer(BlockRenderLayer.TRANSLUCENT);
      
   }

But my liquid still doesn't dim the light like water. I did some research and found that BlockFluidRenderer involved only the vanilla BlockLiquid, but not the BlockFluidClassic class or others from which it is inherited. I looked through many pages in the search, but did not find a similar problem. What do you think about it?

Override getLightOpacity and return something nonzero

I tried, it doesn't work.

Did you make a new world/load new chunks? Lighting doesn’t update automatically IIRC