Here we'll cover creation of a custom fluid. If you plan to create several fluids, it is recommended to make an abstract basic fluid class where you'll set necessary defaults that will be shared in its subclasses. We'll also make it generate in the world like lakes.

Vanilla fluids extend net.minecraft.fluid.BaseFluid class, and so will our abstract fluid. It can be like this:

public abstract class BasicFluid extends BaseFluid
{
    /**
     * @return does it produce infinite fluid (like water)?
     */
    @Override
    protected boolean isInfinite()
    {
        return false;
    }
 
    // make it transparent
    @Override
    protected BlockRenderLayer getRenderLayer()
    {
        return BlockRenderLayer.TRANSLUCENT;
    }
 
    /**
     *
     * @return an associated item that "holds" this fluid
     */
    @Override
    public abstract Item getBucketItem();
 
    /**
     *
     * @return a blockstate of the associated {@linkplain net.minecraft.block.FluidBlock} with {@linkplain net.minecraft.block.FluidBlock#LEVEL}
     */
    @Override
    protected abstract BlockState toBlockState(FluidState var1);
 
    /**
     *
     * @return flowing static instance of this fluid
     */
    @Override
    public abstract Fluid getFlowing();
 
    /**
     *
     * @return still static instance of this fluid
     */
    @Override
    public abstract Fluid getStill();
 
    // how much does the height of the fluid block decreases
    @Override
    protected int getLevelDecreasePerBlock(ViewableWorld var1)
    {
        return 1;
    }
 
    /**
     * 
     * @return update rate
     */
    @Override
    public int getTickRate(ViewableWorld var1)
    {
        return 5;
    }
 
    @Override
    protected float getBlastResistance()
    {
        return 100;
    }
 
    // this seems to determine fluid's spread speed (higher value means faster)
    @Override
    protected int method_15733(ViewableWorld var1)
    {
        return 4;
    }
 
    // I don't know what this does, but it's present in the water fluid
    @Override
    protected void beforeBreakingBlock(IWorld iWorld_1, BlockPos blockPos_1, BlockState blockState_1) {
        BlockEntity blockEntity_1 = blockState_1.getBlock().hasBlockEntity() ? iWorld_1.getBlockEntity(blockPos_1) : null;
        Block.dropStacks(blockState_1, iWorld_1.getWorld(), blockPos_1, blockEntity_1);
    }
 
    // also don't know what it does
    public boolean method_15777(FluidState fluidState_1, BlockView blockView_1, BlockPos blockPos_1, Fluid fluid_1, Direction direction_1) {
        return direction_1 == Direction.DOWN;
    }
 
    /**
     *
     * @return is given fluid instance of this fluid?
     */
    @Override
    public abstract boolean matchesType(Fluid fluid_1);
 
    /**
     * Required for entities to behave in this fluid like in water
     */
    @Override
    public boolean matches(Tag<Fluid> tag_1)
    {
        return tag_1 == FluidTags.WATER;
    }
}