Often version 150 shaders can be simple to convert but this one uses the function texelFetch which is not available in version 120. Leastways my version is not working:

150 version:

in vec4 v_colour;
in vec2 tex_coord;
out vec4 pixel;

uniform sampler2D t0;
uniform float bloom_spread = 1;
uniform float bloom_intensity = 2;

void main() {
    ivec2 size = textureSize(t0, 0);

    float uv_x = tex_coord.x * size.x;
    float uv_y = tex_coord.y * size.y;

    vec4 sum = vec4(0.0);
    for (int n = 0; n < 9; ++n) {
        uv_y = (tex_coord.y * size.y) + (bloom_spread * float(n - 4));
        vec4 h_sum = vec4(0.0);
        h_sum += texelFetch(t0, ivec2(uv_x - (4.0 * bloom_spread), uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x - (3.0 * bloom_spread), uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x - (2.0 * bloom_spread), uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x - bloom_spread, uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x, uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x + bloom_spread, uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x + (2.0 * bloom_spread), uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x + (3.0 * bloom_spread), uv_y), 0);
        h_sum += texelFetch(t0, ivec2(uv_x + (4.0 * bloom_spread), uv_y), 0);
        sum += h_sum / 9.0;
    }

    pixel = texture(t0, tex_coord) - ((sum / 9.0) * bloom_intensity);
}

This is my 120 version but it doesn’t work, it just makes the texture black:

varying vec4 v_color;
varying vec2 v_texCoords;

uniform sampler2D u_texture;
const float bloom_spread = 1.0;
const float bloom_intensity = 2.0;
uniform vec2 size;

void main() {
    // ivec2 size = textureSize(u_texture, 0);

    float uv_x = v_texCoords.x * size.x;
    float uv_y = v_texCoords.y * size.y;

    vec4 sum = vec4(0.0);
    for (int n = 0; n < 9; ++n) {
        uv_y = (v_texCoords.y * size.y) + (bloom_spread * float(n - 4));
        vec4 h_sum = vec4(0.0);
        h_sum += texelFetch(u_texture, ivec2(uv_x - (4.0 * bloom_spread), uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x - (3.0 * bloom_spread), uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x - (2.0 * bloom_spread), uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x - bloom_spread, uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x, uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x + bloom_spread, uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x + (2.0 * bloom_spread), uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x + (3.0 * bloom_spread), uv_y), ivec2(0));
        h_sum += texelFetch(u_texture, ivec2(uv_x + (4.0 * bloom_spread), uv_y), ivec2(0));
        sum += h_sum / 9.0;
    }

    gl_FragColor = texture2D(u_texture, v_texCoords) - ((sum / 9.0) * bloom_intensity);
}

vec4 texelFetch(sampler2D tex, ivec2 size, ivec2 coord) {
    return texture2D(tex, vec2(float(coord.x) / float(size.x), float(coord.y) / float(size.y)));
}