I'm sewing canvus fabric and I'm using 18 needle what should I set my tension on if the bottom thread is loose

With threading tension issues, first you start by checking the bobbin tension and once that is set correctly, everything is done using the top tension dial. This is explains in text article

What is Perfect Sewing Machine Tension? Perfect sewing machine tension occurs when the top and bottom threads are perfectly balanced. That means you shouldn't see any little loops on either the top or bottom of your fabric and the stitching should look even on both sides.

Sewing Machine Tension - Top 10 TIPS - Treasurie blog

The two top videos are excellent at explaining how to set bobbin tension, and how to diagnose where the issue is, top or bottom?

https://www.google.com/search?q=sewing+machine+tension

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try this link.
Sewing Machines The thread is loose at bottom Google Search

Factory standard tension for upper thread is the midway point between the high and low number. However, that tension dial is meant to be used for tweaking tension. Generally, tweaking is required whenever the type of fabric, needle, or thread are changed, ie thicker fabric like denim should have a different upper tension setting than fine chiffon. To be successful with tension settings, one must become comfortable adjusting the upper tension.

NOTE: Accurate tension is achieved when the upper thread and bobbin thread meet in the middle of the fabric.
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Understanding Bobbin Tension


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Make sure you raise the presser foot BEFORE threading the upper thread so it will seat properly in the tension disk.

The factory standard tension setting is the halfway point between the low & high numbers. Be sure you are using needle & thread that are compatible with each other and the fabric you are sewing.
All About Needles

Test your stitch on the fabric you are sewing. If the upper thread is showing on the bottom of the fabric, the upper tension is too loose. If the bobbin thread is showing on the top of the fabric, the upper tension is too tight.

Ideal tension is when both upper and bobbin threads meet in the middle of the fabric. The tension will need adjusting anytime you change fabric weight, needle size, and thread weight.

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use a couple of pieces of waste material the same as the sweat pants material and sew about an inch long
adjust the needle tension and do it again
repeat the exercise until the top and bottom thread joint at the middle of the two pieces of the material
tension settings on the machine are a loose guide as the thread thickness, material thickness and needle size all have a bearing in the tensions of the thread
so practice and adjust prior to sewing is the way to go

This is a problem with thread tension. The needle thread is too tight and the bobbin thread is too loose. I don't have one of these machines, but there should be either screws or knobs to adjust. Look at your manual. Here's a page I found on bobbin tension.
https://www.manualslib.com/manual/1022927/Toyota-Quilt-50.html?page=50

A. To correct loose stitches, try the following: (1) Balance the thread tension. If the stitches are loose on top of the fabric, decrease the tension to the next lower number. If they are loose on the underside set the tension to the next higher number. (2) Make sure the needle/upper thread follows the threading path. (3) Check to see that the bobbin case is threaded properly. (4) Use a different size needle. For fine fabrics, use a smaller needle (size 9, 11); medium-weight fabrics require a size 14 or 16. And use a size 18 or "Denim" needle for heavy corduroy or denim.

This is a tension adjustment problem. It's good to use two different colors of thread, like you did. From your description, the top thread (needle thread) tension needs to be tighter. As you tighten the tension of the top thread, you'll see that the top thread pulls the bobbin thread up into the fabric so it's no longer laying loose on the bottom of the fabric.

You'll need to have the pressure foot down when you tighten the tension. Tighten it a little, sew a practice seam, check to see what the stitch looks like. Tighten it a little more, sew, check, etc etc--until both top and bottom look the same--you shouldn't see much (if any) of the bobbin thread color on top of the fabric or the top thread color on the underside.

Let me know how this works for you, ok?


Robbie

Unthread your machine and set the tension to 4.5. rethread your machine up to the needle and then put the presser foot down. If the thread pulls hard, your machine is threaded right if it pulls easily, the thread is not thru the tension wheel correctly.

Rule of thumb:: loops on the top, problem on the bottom.
loops on the bottom, problem on the top.

If it was sewing correctly BEFORE the needle break...why did the needle break in the first place?...and what fixes did you make?
If it sewed properly...what changed?

Cut your upper thread close to the spool...make sure the presser foot is raised and pull the cut off thread section through/out of the machine from the needle area (sewing direction...do not pull the thread backwards!)

Clean out the bobbin area...remove any traces of lint or pieces of thread...look for any broken off needle pieces... brush the bobbin area out....then add a drop of sewing machine oil (but only if your manual says to oil that area).

Remove the needle plate to expose the feed dogs...brush/clean out that area too and add a drop of sewing machine oil...then secure the needle plate back on.
Replace the needle...do it again...yup...some needles arrive bad from the factory. Make sure the needle is inserted and positioned properly. Then thread the machine WITH the presser foot in raised position. (The thread needs to get seated into the upper tensions.)
..with the.presser foot down...thread the new needle.
Oh...and use the correct needle for the thread. (a universal 80/12 is standard).
Thread can cause issues too. Try a different bobbin, or maybe a different spool of thread to see if that makes a difference.