Access Query Problem

You are comparing strings, not numeric values, and all your field values seem to be text, thus should be converted. Try this:

SELECT
    CT_ALL.CTDate,
    CT_ALL.[12AM],
    CT_ALL.[4AM],
    CT_ALL.[8AM],
    CT_ALL.NOON,
    CT_ALL.[4PM],
    CT_ALL.[8PM],
    CT_ALL.High,
    CT_ALL.CT1_Time,
    CT_ALL.Peak,
    CT_ALL.CT2_Time,
    CT_ALL.C,
    CT_ALL.T,    
    Val(Format(CDbl(Nz([C], 1)) * CDbl([T]), '#.0')) AS CxT,
    CT_ALL.Sed_Cl2,
    CT_ALL.Sed_T,
    CT_ALL.Temp,
    CT_ALL.pH,
    CT_ALL.[3 Log],
    CT_ALL.Log,
    Val(Format(CDbl(Nz([C2], 0)) * CDbl([T2]), '#.0')) AS CxT2,
    CT_ALL.Sed_Cl2 AS C2,
    CT_ALL.Sed_T AS T2,
    IIf(
        [Temp_1] = '--',
        Null,
        IIf(
            Val([temp]) < 12.5,
            (0.353 * 0.5) * (12.006 + 2.71828183 ^ (2.46 - 0.073 * Val([Temp]) + 0.125 * Val([C]) + 0.389 * Val([pH]))),
            (0.361 * 0.5) * (-2.261 + 2.71828183 ^ (2.69 - 0.065 * Val([Temp]) + 0.111 * Val([C]) + 0.361 * Val([pH])))
        )
    ) AS CT_Required,
    Nz([Temp], '') AS Temp_1,
    CT_ALL.C AS C1,
    IIf([CxT] > [Ct_Required], 'YES', 'NO') AS ClearwellMet,
    IIf([CxT_Sum] > [CT_Required], 'YES', 'NO') AS Met_Overall,
    IIf(
        [temp_1] = '--',
        Null,
        IIf(
            Val([temp]) < 12.5,
            (0.353 * 3) * (12.006 + 2.71828183 ^ (2.46 - 0.073 * Val([Temp]) + 0.125 * Val([C]) + 0.389 * Val([pH]))),
            (0.361 * 3) * (-2.261 + 2.71828183 ^ (2.69 - 0.065 * Val([Temp]) + 0.111 * Val([C]) + 0.361 * Val([pH])))
        )
    ) AS 3Log
FROM CT_ALL
WHERE CT_ALL.CTDate Between [forms].[FiltrationPlant].[StartDate]
    And DateAdd('m', 1, ([Forms].[FiltrationPlant].[StartDate])) - 1;