Buoyancy question

I saw an ornament like that a while back, a submerged refugium for crabs, frogs, and salamanders. I was going to recommend a glass jar but maybe a glass bowl would work better? The weight of the glass might be enough.

If you want the science answer. You need to know the empty weight of the jar preferably in grams and measure the volume of the jar preferably in cubic centimeters. (metric keeps math simple)

Anyway you would then add weight to the jar until it exceeds a ratio of about 1 gram per cubic centimeter. The density of pure water at 20°C or 68°F is about 0.998 grams per cubic centimeter and is about 0.9999 grams per cubic centimeter at 4°C or 39.2°F. Anything with a density greater than water will sink and anything with a density lower than water will float.

This is the how those pretty Galileo Thermometers you see at Walmart work.
They have a number of sealed glass bulbs filled with a colored liquid. The density of each bulb is slightly different corresponding to the density of water at a different temperature. As the temperature of the liquid they are submerged in changes, the bulbs that are less dense than the water float and those with higher density sink.

There are a couple of ways you can measure the volume of the jar. The easy but less accurate way is to just measure the dimensions of the jar and calculate the volume. This is difficult for objects of odd shape.
The formula for the volume of a cylinder is:
Volume = (Diameter/2 * pie) Squared * Height.

The more accurate method to determine the volume of any object is to submerge it in water and measure the amount of water it displaces. 1mL = 1 cubic centimeter. This works with objects of any shape.

The most accurate method involves using a digtal balance to measure the weight of an object hanging from a small string dry, then submerge it in water without allowing it to touch the sides or bottom and noting the difference in weight. If your weighing the object the weight will decrease by an amount equal to the weight of the water the volume of the object is displacing. If you are weighing the water you are submerging the object in the weight will increase by the weight of the water the volume of the object is displacing.