Welcome to Tropical Fish Keeping forum.
Knowing the present fixture data would help. If it is a single-tube fluorescent with a 24-inch tube, and you want to stay with it (rather than another fixture), I would suggest a Life-Glo tube. These are made by Hagen/Nutrafin, and are expensive, but they have a coating inside half the tube that directs the light more intensely so they have higher light production than other tubes. There are two Life-Glo, the Life-Glo 2 is "ordinary" but the Life-Glo [with no number] is the special one. It is an excellent plant and aquarium tube. Tubes need replacing every 2-3 years.
There are a couple issues with your present fish I should mention. All loaches are social fish and must be kept in groups, 5-6 or more. They will be under stress otherwise. However, given the mature size of Clown Loach at 8-12 inches, this is not possible in anything under a 5-6 foot tank. The small tank (to them) is also a cause for stress, and stress cause innumerable health issues that would otherwise not occur. You can read more in our profile of this fish; click on the shaded name in posts, or use the second tab from the left in the blue bar at the top.
Serpae Tetra like all tetra are also shoaling fish tat need a group; in this particular species, it can be very aggressive when kept in less than 8. Again, more info is in the profile. And while this or that particular fish may "appear" fine, this can suddenly change. A recent study has proven that shoaling fish maintained in groups under 5 will most likely exhibit increased aggression over time due to the stress of their environment.